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#include <iostream>
#include <cstdio>
#include <algorithm>
#include <vector>

using namespace std;

vector <int> A[500005];
bool visited[500005];
bool flag = false;
bool dfs( int v, int block )
{
    visited[v] = true;
    for( vector <int> :: iterator it = A[v].begin(); it != A[v].end(); it ++ )
    {
        if( *it == block ) continue;
        if( visited[*it] == false )
        {
            dfs(*it,block);
        }
        else
        {
            flag = true;
            break;
        }
    }
    return flag;
}

void clean( int n )
{
    for( int i = 1; i <= n; i ++ )
    {
        visited[i] = false;
    }
}

int main()
{
    int n, m;
    scanf("%d %d", &n, &m);
    for( int i = 0; i < m; i ++ )
    {
        int a, b;
        scanf("%d %d", &a, &b);
        A[a].push_back(b);
    }
    bool g = false;
    for( int i = 1; i < n; i ++ )
    {
        flag = false;
        clean(n);
        if( dfs(i,0) == true ) { g = true; break; }
    }
    if( g == false )
    {
        printf("NIE\n");
        return 0;
    }
    //if( dfs(1,2) == true ) cout << "jest cykl\n";
    //else cout << "ni ma\n";
    vector <int> L;
    for( int i = 1; i <= n; i ++ )
    {
        bool K = true;
        for( int j = 1; j <= n; j ++ )
        {
            flag = false;
            clean(n+1);
            visited[i] = true;
            if( j != i && visited[j] == false )
            {
                bool f = dfs(j,i);
                if( f == true )
                {
                    K = false;
                    break;
                }
            }
        }
        if( K == true ) L.push_back(i);
    }
    if( L.size() == 0 ) { printf("NIE\n"); return 0; }
    sort(L.begin(),L.end());
    printf("%d\n", L.size());
    for( int i = 0; i < L.size(); i ++ )
    {
        printf("%d ", L[i]);
    }
    return 0;
}