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#include <bits/stdc++.h>

using namespace std;

typedef long long LL;

template<typename TH>
void debug_vars(const char* data, TH head){
    cerr << data << "=" << head << "\n";
}

template<typename TH, typename... TA>
void debug_vars(const char* data, TH head, TA... tail){
    while(*data != ',') cerr << *data++;
    cerr << "=" << head << ",";
    debug_vars(data+1, tail...);
}

#ifdef LOCAL
#define debug(...) debug_vars(#__VA_ARGS__, __VA_ARGS__)
#else
#define debug(...) (__VA_ARGS__)
#endif

/////////////////////////////////////////////////////////


const int MaxN = 500005,
          MaxM = 1000005;

int N, M;
vector<int> adj[MaxM], revAdj[MaxM];
int cycPos[MaxN];
bool visited[MaxN], current[MaxN];
bool reachable[MaxN];

void input(){
    scanf("%d%d", &N, &M);
    for(int i = 0; i < M; i++){
        int u, v;
        scanf("%d%d", &u, &v);
        adj[u].push_back(v);
        revAdj[v].push_back(u);
    }
}

vector<int> cycle;
bool cnt;

bool dfs(int v){
    visited[v] = current[v] = true;

    for(int s : adj[v]){
        if(current[s]){
            cnt = false;
            cycle.push_back(s); return true;
        }
        if(visited[s]) continue;

        if(dfs(s)){
            cycle.push_back(s);
            return (cycle[0] != v);
        }
        if(!cnt) return false;
    }

    current[v] = false;
    return false;
}

vector<int> find_cycle(){
    cnt = true;
    fill(current, current+N+1, false);
    cycle.clear();
    for(int i = 1; i <= N; i++){
        if(!visited[i]){
            dfs(i);
            if(cycle.size() > 0) break;
        }
    }
    return cycle;
}

int main(){
    input();
    vector<int> cyc = find_cycle();
    if(cyc.size() == 0){
        printf("NIE\n");
        return 0;
    }
    reverse(cyc.begin(), cyc.end());
    fill(visited, visited+N+1, false);
    for(int i = 0; i < (int)cyc.size(); i++){
        visited[cyc[i]] = true;
        cycPos[cyc[i]] = i+1;
    }


    if(find_cycle().size() > 0){
        printf("0\n\n");
        return 0;
    }

    cyc.insert(cyc.begin(), -1);
    fill(reachable, reachable+N+1, false);

    // znajdujemy najbardziej na lewo sciezke idaca w lewo
    int L = -1, R = N+100;
    for(int id = 1; id < (int)cyc.size(); id++){
        int v = cyc[id];

        queue<int> Q;
        Q.push(v);
        while(!Q.empty()){
            int s = Q.front(); Q.pop();
            for(int o : revAdj[s]){
                if(reachable[o]) continue;
                reachable[o] = true;
                if(!cycPos[o]) Q.push(o);
            }
        }
        if(reachable[v]){ R = id; break; }
    }

    // znajdujemy najbardziej na prawo sciezke idaca w lewo
    fill(reachable, reachable+N+1, false);
    for(int id = (int)cyc.size()-1; id > 0; id--){
        int v = cyc[id];

        queue<int> Q; Q.push(v);
        while(!Q.empty()){
            int s = Q.front(); Q.pop();
            for(int o : adj[s]){
                if(reachable[o]) continue;
                reachable[o] = true;
                if(!cycPos[o]) Q.push(o);
            }
        }
        if(reachable[v]){ L = id; break; }
    }

    // teraz wszystkie odpowiedzi musza byc wewntrz [L,R]
    if(L > R){
        printf("0\n\n"); return 0;
    }

    // znowu robimy cos podobnego, ale znow troszke inaczej
    fill(reachable, reachable+N+1, false);
    vector<int> result;
    int minCorrect = 0;
    for(int id = 1; id < (int)cyc.size(); id++){
        int v = cyc[id];
        if(id >= minCorrect && id >= L && id <= R) result.push_back(v);

        queue<int> Q; Q.push(v);
        while(!Q.empty()){
            int s = Q.front(); Q.pop();
            for(int o : adj[s]){
                if(reachable[o]) continue;
                reachable[o] = true;
                if(cycPos[o]){
                    minCorrect = max(minCorrect, cycPos[o]);
                } else {
                    Q.push(o);
                }
            }
        }
    }

    sort(result.begin(), result.end());
    printf("%zu\n", result.size());
    for(int v : result) printf("%d ", v);
    printf("\n");
}