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#include <vector>
#include <stack>
#include <cstdio>
#include <queue>

using namespace std;
typedef long long ll;

ll n;
ll m;
vector<long long> graph[500003];
int was[500003];
ll count[500003];
ll numofcycles;
stack<long long> mystack;
queue<int> myqueue;
/*
void clear()
{
	for(int i = 1; i <= n; i++) was[i] = 0;
}
*/

void stackupdate()
{
	numofcycles++;
	
	stack<long long> pom;
	
	while(!mystack.empty())
	{
		ll t = mystack.top();
		mystack.pop();
		count[t]++;
		pom.push(t);
	}
	
	while(!pom.empty())
	{
		ll t = pom.top();
		pom.pop();
		mystack.push(t);	
	}
	
	//jest takie jak bylo			
}

void dfs(ll x)
{
	
	was[x] = 1;
	mystack.push(x);
	
	ll s = graph[x].size();
	
	for(int i = 0; i < s; i++)
	{
		
	
		if(was[graph[x][i]] == 1)
		{
			stackupdate();	
		}
		else
		{
			dfs(graph[x][i]);	
		}
	}
	
	was[x] = 0;
	mystack.pop();
	
}


int main()
{
	scanf("%lld", &n);
	scanf("%lld", &m);
	ll a,b;
	
	for(int i = 0; i < m; i++)
	{
		scanf("%lld %lld", &a, &b);
		graph[a].push_back(b);
	}
	
	for(int i = 1; i <= n; i++)
	{
		//clear();
		dfs(i);
	}
	
	
	
	if(numofcycles == 0)
	{
		printf("NIE\n");
		return 0;
	}
	
	
	ll k = 0;
	
	for(int i = 1; i <= n; i++)
	{
		if(count[i] == numofcycles)
		{
			k++;
			myqueue.push(i);
		}
	}
	
	if(k == 0)
	{
		printf("NIE\n");
		return 0;
	}
	
	
	
	printf("%lld\n", k);
	
	while(!myqueue.empty())
	{
		int pp = myqueue.front();
		myqueue.pop();
		printf("%d ", pp);
	}
	
	printf("\n");
	
	return 0;
}