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#include <cstdlib>
#include <iostream>
#include <vector>

using namespace std;

struct FromTo
{
   int From;
   int To;
};

enum CityType
{
   normal = 0,
   process = 1,
   critic = 2,
//   cycle = 3,
//   oneway = 4,
   blank = 5,
//   external = 6
};

struct City
{
   City() : group(normal), next(-1) {}
   CityType group; //0 - not founded yet; 1 - current process; 2 - possible critic; 3 - cycle, but not critic; 4 - 1 way connected to cycle; 5 - blank path
   int count;
   vector<int> neight;
   int next;
};

vector<FromTo> excluded;

City Cities[500001];
int Map[500001];
int RevMap[500001];
int zlicz[500001];
int CycleStart = -1;
int CurrentStart = -1;
bool CycleFirstDepth = false;

int SearchCycle(int idx) //return 0 - blank, 1 - cycle found but not for you, 2 - found cycle
{
   if(Cities[idx].group == process)
   {
      Cities[idx].group = critic;
      CycleStart = idx;
      return 2;
   }
   
   if(Cities[idx].group == blank)
      return 0;
   
   Cities[idx].group = process;
   
   for(vector<int>::iterator it = Cities[idx].neight.begin(); it != Cities[idx].neight.end(); it++)
   {
      int result = SearchCycle(*it);
      if(result == 0) //not cycle found
         continue;
         
      if(result == 1) // cycle found, but currently not on critic cycle
      {
         Cities[idx].group = normal;
         return 1;
      }
      
      //result == 2 //critic cycle
      Cities[idx].next = *it;
      if(Cities[idx].group == critic)
         return 1;
         
      Cities[idx].group = critic;
      return 2;
   }
   
   Cities[idx].group = blank;
   return 0;
}

int SearchOtherCycle(int idx) // 0 - blank; 1 - cycle (different than critic)
{
   if(Cities[CurrentStart].next == idx)
      return 0;
      
   if(CycleFirstDepth == false && Cities[idx].group == critic)
   {
      if(CurrentStart != -1)
      {
         FromTo ft;
         ft.From = CurrentStart;
         ft.To = idx;
         excluded.push_back(ft);
      }
      
      return 0;
   }
   CycleFirstDepth = false;
   
   if(Cities[idx].group == blank)
      return 0;
   
   if(Cities[idx].group == process)//found other cycle
      return 1;
   
   if(Cities[idx].group == normal)
      Cities[idx].group = process;
   
   for(vector<int>::iterator it = Cities[idx].neight.begin(); it != Cities[idx].neight.end(); it++)
   {
      if(SearchOtherCycle(*it) == 1)
         return 1;
   }
   
   if(Cities[idx].group == process)
      Cities[idx].group = blank;
   return 0;
}

int main(int argc, char *argv[])
{
   int n,m;
   cin>>n>>m;
   for(int i = 0; i < m; i++)
   {
      int a,b;
      cin>>a>>b;
      
      Cities[a].neight.push_back(b);
   }
   
   for(int i = 1; i <= n; i++)
   {
      if(Cities[i].group != normal)
         continue;
         
      if(SearchCycle(i) != 0)
         break;
   }
   
   if(CycleStart == -1)
   {
      cout<<"NIE";
      return 0;
   }
   
   bool first = true;
   for(CurrentStart = CycleStart; CurrentStart != CycleStart || first; CurrentStart = Cities[CurrentStart].next)
   {
      first = false;
      CycleFirstDepth = true;
      if(SearchOtherCycle(CurrentStart) == 1)
      {
         cout<<"0"<<endl;
         return 0;
      }
   }
   
   CurrentStart = -1;
   for(int i = 1; i <= n; i++)
   {
      if(Cities[i].group != normal)
         continue;
         
      if(SearchOtherCycle(i) == 1)
      {
         cout<<"0"<<endl;
         return 0;
      }
   }
   
   //we need to remove from critic cycle all founded noncycle
   //create map
   first = true;
   int lenght = 0;
   for(CurrentStart = CycleStart; CurrentStart != CycleStart || first; CurrentStart = Cities[CurrentStart].next)
   {
      first = false;
      RevMap[lenght] = CurrentStart;
      Map[CurrentStart] = lenght++;
   }
   
   for(vector<FromTo>::iterator it = excluded.begin(); it != excluded.end(); it++)
   {
      int mTo = Map[it->To];
      int mFrom = Map[it->From];
      int diff = (lenght + mTo - mFrom) % lenght;
      if(diff == 0)
         diff = lenght;
      if(zlicz[mFrom] < diff)
         zlicz[mFrom] = diff;
   }
   
   //mark uncritical as blank
   int remains = 0;
   for(int i = 0; i < lenght; i++)
   {
      if(remains > 0)
         Cities[RevMap[i]].group = blank;
         
      if(remains < zlicz[i])
         remains = zlicz[i];
         
      remains--;
   }
   for(int i = 0; remains > 0 && i < lenght; i++)
   {
      Cities[RevMap[i]].group = blank;
      remains--;
   }
   
   int Remaining = 0;
   first = true;
   for(CurrentStart = CycleStart; CurrentStart != CycleStart || first; CurrentStart = Cities[CurrentStart].next)
   {
      first = false;
      if(Cities[CurrentStart].group == critic)
         Remaining++;
   }
   
   cout<<Remaining<<endl;
   
   for(int i = 1; i <= n; i++)
   {
      if(Cities[i].group == critic)
         cout<<i<<" ";
   }
   
   
   
   
   /*
   
   
   for(int i = 1; i <= n; i++)
   {
      Cities[i].count = Cities[i].neight.size();
      if(Cities[i].count < d)
      {
         Cities[i].group = -1;
         ToProcess.push(i);
      }
   }
   
   //part one remove cities with neightbours less than d
   while(ToProcess.empty() == false)
   {
      int a = ToProcess.front();
      for(vector<int>::iterator it = Cities[a].neight.begin(); it != Cities[a].neight.end(); ++it)
      {
         int b = *it;
         if(Cities[b].group != 0) //it is or was on ToProcess queue
            continue;
         
         Cities[b].count--;
         if(Cities[b].count < d)
         {
            Cities[b].group = -1;
            ToProcess.push(b);
         }
      }
      ToProcess.pop();
   }
   
   //all cities have minimum d roads
   //lets split this to group
   int maxGroup = 0;
   int maxGroupSize = 0;
   int CurrentGroup = 1;
   
   for(int s = 1; s <= n; s++)
   {
      if(Cities[s].group != 0)
         continue;
      
      Cities[s].group = CurrentGroup;
      ToProcess.push(s);
      
      int CurrentGroupSize = 0;
      while(ToProcess.empty() == false)
      {
         CurrentGroupSize++;
         int a = ToProcess.front();
         for(vector<int>::iterator it = Cities[a].neight.begin(); it != Cities[a].neight.end(); ++it)
         {
            int b = *it;
            if(Cities[b].group != 0) //it is or was on ToProcess queue
               continue;
            
            Cities[b].group = CurrentGroup;
            ToProcess.push(b);
         }
         ToProcess.pop();
      }
      
      if(CurrentGroupSize > maxGroupSize)
      {
         maxGroupSize = CurrentGroupSize;
         maxGroup = CurrentGroup;
      }
      
      CurrentGroup++;
   }
   
   if(maxGroupSize == 0)
   {
      cout<<"NIE";
      return 0;
   }
   
   cout<<maxGroupSize<<endl;
   
   vector<int> Final;
   for(int i = 1; i <= n; i++)
   {
      if(Cities[i].group == maxGroup)
         Final.push_back(i);
   }
   
   for(vector<int>::iterator it = Final.begin(); it != Final.end(); ++it)
   {
      cout<<*it<<" ";
   }
   
   cin>>n;*/
}