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#include <iostream>
#include <cstdio>
#include <algorithm>
#include <vector>
#include <map>
#include <queue>
#include <set>
#include <cstring>
#include <stack>

using namespace std;

const int N = 500050;
const int MX=1000000000;

vector<int> V[N];
int vis[N];
int n,m;
int ft,st;

int ic=0;

stack<int> ST;
vector<int> res;

int main()
{
    scanf("%d%d",&n,&m);
    int a,b;
    for(int i=0;i<m;++i) {
        scanf("%d%d",&a,&b);
        V[a-1].push_back(b-1);
    }
    ic=0;
    memset(vis,0,sizeof(vis));
    for(int i=0;i<n;++i) {
        ST.push(i);
        vis[i]=1;
        while(!ST.empty()) {
            int a = ST.top();
            ST.pop();
            for(int j=0;j<V[a].size();++j) {
                int d=V[a][j];
                if (d==i) {
                    ft = i;
                    ic = 1;
                    break;
                }
                if (vis[d]) continue;
                vis[d]=1;
                ST.push(d);
            }
            if (ic) {
                while(!ST.empty())
                    ST.pop();
                break;
            }
        }
        if (ic) break;
    }
    if (!ic) {
        printf("NIE\n");
        return 0;
    }
    ic=0;
    memset(vis,0,sizeof(vis));
    for(int i=0;i<n;++i) {
        if (i==ft) continue;
        ST.push(i);
        vis[i]=1;
        while(!ST.empty()) {
            int a = ST.top();
            ST.pop();
            for(int j=0;j<V[a].size();++j) {
                int d=V[a][j];
                if (d==i) {
                    st = i;
                    ic = 1;
                    break;
                }
                if (vis[d]) continue;
                vis[d]=1;
                ST.push(d);
            }
            if (ic) {
                while(!ST.empty())
                    ST.pop();
                break;
            }
        }
        if (ic) break;
    }


    for(int i=0;i<n;++i) {
        memset(vis,0,sizeof(vis));
        int act=(i==ft?st:ft);
        vis[act]=1;
        ST.push(act);
        ic=0;
        while(!ST.empty()) {
            int a = ST.top();
            ST.pop();
            for(int j=0;j<V[a].size();++j) {
                int d=V[a][j];
                if (d==act) {
                    ic = 1;
                    break;
                }
                if (d==i) continue;
                if (vis[d]) continue;
                vis[d]=1;
                ST.push(d);
            }
            if (ic) {
                while(!ST.empty())
                    ST.pop();
                break;
            }
        }
        if (!ic) {
            res.push_back(i+1);
        }
    }


    printf("%d\n",res.size());
    for(int i=0;i<res.size();++i)
        printf("%d ",res[i]);



    return 0;
}