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#include <cstdio>
#include <vector>
#define scanf(args...) (scanf(args) ? : 0)
const int MAXN = 500005;

std::vector<int> adj[MAXN];
bool visited[MAXN];
int value[MAXN];
int edge_count[MAXN];
int upper_edge_count;

void dfs(int u) {
    if (visited[u])
        return;
    visited[u] = true;
    for (int v: adj[u])
        if (!visited[v]) {
            dfs(v);
            value[u] += value[v] - edge_count[v];
        }
        else {
            value[u]++;
            edge_count[v]++;
            upper_edge_count++;
        }
}

int main() {
    int n, m;
    scanf("%d%d", &n, &m);

    for (int i = 0; i < m; i++) {
        int u, v;
        scanf("%d%d", &u, &v);
        u--, v--;
        adj[u].push_back(v);
    }

    for (int i = 0; i < n; i++)
        dfs(i);
    
    std::vector<int> result;
    for (int i = 0; i < n; i++) {
        if (value[i] == upper_edge_count)
            result.push_back(i);
    }

    if (upper_edge_count == 0)
        printf("NIE\n");
    else {
        printf("%d\n", (int)result.size());
        for (int i = 0; i < result.size(); i++)
            printf("%d ", result[i] + 1);
        printf("\n");
    }
}