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#include <cstdio>
#include <vector>
using namespace std;

struct Node{
    vector<int> outedges;
    int t;
    bool available=true;
};

int n,m;
Node g[500009];

int topo;
void Dfs(int v){
    if(!g[v].t && g[v].available){
        g[v].t=1;
        for(int e : g[v].outedges) Dfs(e);
        g[v].t = --topo;
    }
}

void TopoSort(){
    for(int i=1;i<=n;++i) g[i].t=0;
    topo = n;
    for(int i=1;i<=n;++i) Dfs(i);
}

bool isAcyclic(){
    TopoSort();
    for(int i=1;i<=n;++i){
        if(!g[i].available) continue;
        for(int e : g[i].outedges){
            if(g[e].available && g[i].t >= g[e].t)
                return false;
        }
    }
    return true;
}

int main(){
    scanf("%d%d",&n,&m);
    for(int i=0;i<m;++i){
        int a,b;
        scanf("%d%d",&a,&b);
        g[a].outedges.push_back(b);
    }
    if(isAcyclic()){
        puts("NIE");
    }else{
        vector<int> pewne;
        for(int i=1;i<=n;++i){
            g[i].available = false;
            if(isAcyclic()) pewne.push_back(i);
            g[i].available = true;
        }
        printf("%d\n",pewne.size());
        for(int i=0;i<pewne.size();++i){
            printf("%d ",pewne[i]);
        }
        puts("");
    }
    return 0;
}