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// Artur Kraska, II UWr

#include <algorithm>
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <vector>
#include <cmath>
#include <list>
#include <set>
#include <map>

#define forr(i, n)                  for(int i=0; i<n; i++)
#define FOREACH(iter, coll)         for(auto iter = coll.begin(); iter != coll.end(); ++iter)
#define FOREACHR(iter, coll)        for(auto iter = coll.rbegin(); iter != coll.rend(); ++iter)
#define lbound(P,R,PRED)            ({typeof(P) X=P,RRR=(R), PPP = P; while(PPP<RRR) {X = (PPP+(RRR-PPP)/2); if(PRED) RRR = X; else PPP = X+1;} PPP;})
#define testy()                     int _tests; scanf("%d", &_tests); FOR(_test, 1, _tests)
#define CLEAR(tab)                  memset(tab, 0, sizeof(tab))
#define CONTAIN(el, coll)           (coll.find(el) != coll.end())
#define FOR(i, a, b)                for(int i=a; i<=b; i++)
#define FORD(i, a, b)               for(int i=a; i>=b; i--)
#define MP                          make_pair
#define PB                          push_back
#define deb(X)                      X;

#define M 1000000007
#define INF 1000000007

using namespace std;

int n, m, numer = 1, a, b, ilosc;
vector <int> v, res;
list <int> cykl;
bool jest;
int byl_nr, teraz_nr;
int pocz, kon, pocz_ok, kon_ok;

struct miasto
{
    int byl;
    bool ok;
    bool jest;
    list <int> l;
};
miasto tab[1000007];

bool dfs(int nr)
{
//    cout << "dfs1 wchodzi do " << nr << endl;
    if(tab[nr].byl == byl_nr)
    {
        if(tab[nr].jest)
        {
            pocz = nr;
            cykl.PB(nr);
            tab[nr].ok = 1;
            return 1;
        }
        return 0;
    }
    tab[nr].jest = 1;
    tab[nr].byl = byl_nr;


    FOREACH(it, tab[nr].l)
    {
        if(dfs(*it))
        {
            if(nr == pocz)
            {
                pocz = -1;
            }
            else if(pocz != -1)
            {
                tab[nr].ok = 1;
                cykl.PB(nr);
            }
            return 1;
        }
    }
    tab[nr].jest = 0;

    return 0;
}

bool dfs2(int nr, int zly)
{
    if(nr == zly)
        return 0;

    if(tab[nr].byl == byl_nr)
    {
        if(tab[nr].jest)
        {
            return 1;
        }
        return 0;
    }
    tab[nr].jest = 1;
    tab[nr].byl = byl_nr;


    FOREACH(it, tab[nr].l)
    {
        if(dfs2(*it, zly))
        {
            return 1;
        }
    }
    tab[nr].jest = 0;

    return 0;
}

int main()
{
    v.PB(0);

    scanf("%d %d", &n, &m);
    forr(i, m)
    {
        scanf("%d %d", &a, &b);
        tab[a].l.PB(b);
    }

    bool jest = 0;
    FOR(i, 1, n)
        if(tab[i].byl == 0)
        {
            byl_nr++;

            jest = dfs(i);
            if(jest)
                break;
        }


    FOREACH(it, cykl)
    {
//        cout << " na cyklu: " << (*it) << endl;
        byl_nr++;
        FOR(i, 1, n)
        {
            if(tab[*it].byl != byl_nr)
                if(dfs2(i, *it))
                {
                    tab[*it].ok = 0;
                    break;
                }
        }
    }

    if(!jest)
    {
        printf("NIE\n");
        return 0;
    }

    FOR(i, 1, n)
    {
        if(tab[i].ok)
            res.PB(i);
    }
    printf("%d\n", res.size());
    FOREACH(it, res)
    {
        printf("%d ", *it);
    }
    printf("\n");

    return 0;
}