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/*

Using algorithm described by R. Tarjan, Enumeration of the elementary circuits of a directed graph, SIAM Journal on Computing, 2 (1973), pp. 211-216

Implementation based on:
- https://github.com/josch/cycles_tarjan/blob/master/cycles.py

*/

#include <cstdio>
#include <algorithm>
#include <functional>
#include <vector>

using namespace std;

const int MAX_N = 500000;
const int MAX_M = 1000000;

vector<int> adj[MAX_N];
vector<int> adj_tr[MAX_N];
vector<int> adj_scc[MAX_N];
bool visited[MAX_N];
vector<int> f_order;
int scc[MAX_N];
int scc_cnt[MAX_N];
int relabel_to[MAX_N];
int relabel_from[MAX_N];
int cycle_cnt[MAX_N];
int num_cycles;

int s;
vector<int> point_stack;
bool marked[MAX_N];
vector<int> marked_stack;

void dfs_visit(int u) {
    visited[u] = true;
    for (int i = 0; i < adj[u].size(); i++) {
        int v = adj[u][i];
        if (!visited[v]) {
            dfs_visit(v);
        }
    }
    f_order.push_back(u);
}

void dfs_visit_tr(int u, int curr_scc) {
    scc[u] = curr_scc;
    visited[u] = true;
    for (int i = 0; i < adj_tr[u].size(); i++) {
        int v = adj_tr[u][i];
        if (!visited[v]) {
            dfs_visit_tr(v, curr_scc);
        }
    }
}

void dfs(int n) {
    for (int u = 0; u < n; u++) {
        visited[u] = false;
    }
    for (int u = 0; u < n; u++) {
        if (!visited[u]) {
            dfs_visit(u);
        }
    }
}

void dfs_tr(int n) {
    for (int u = 0; u < n; u++) {
        visited[u] = false;
    }
    int curr_scc = 0;
    for (int i = n - 1; i >= 0; i--) {
        int u = f_order[i];
        if (!visited[u]) {
            dfs_visit_tr(u, curr_scc++);
        }
    }
}

void strongly_connected_components(int n) {
    dfs(n);
    dfs_tr(n);
}

bool backtrack(int v) {
    bool f = false;
    point_stack.push_back(v);
    marked[v] = true;
    marked_stack.push_back(v);
    for (int i = 0; i < adj_scc[v].size(); i++) {
        int w = adj_scc[v][i];
        if (w != -1) {
            if (w < s) {
                adj_scc[v][i] = -1;
            }
            else if (w == s) {
                //printf("Cycle: ");
                num_cycles++;
                for (int j = 0; j < point_stack.size(); j++) {
                    //printf("%d ", point_stack[j]);
                    cycle_cnt[point_stack[j]]++;
                }
                //printf("\n");
                f = true;
            }
            else if (!marked[w]) {
                f = ((backtrack(w)) || (f));
            }
        }
    }
    if (f) {
        while (marked_stack.back() != v) {
            int u = marked_stack.back();
            marked_stack.pop_back();
            marked[u] = false;
        }
        marked_stack.pop_back();
        marked[v] = false;
    }
    point_stack.pop_back();
    return f;
}

int main() {
    int n, m, a, b;
    scanf("%d%d", &n, &m);
    for (int i = 0; i < m; i++) {
        scanf("%d%d", &a, &b);
        adj[a - 1].push_back(b - 1);
        adj_tr[b - 1].push_back(a - 1);
    }

    strongly_connected_components(n);
    for (int i = 0; i < n; i++) {
        scc_cnt[i] = 0;
    } 
    for (int i = 0; i < n; i++) {
        scc_cnt[scc[i]]++;
    }
    int num_scc = 0;
    int check_scc = -1;
    for (int i = 0; i < n; i++) {
        if (scc_cnt[i] > 1) {
            num_scc++;
            check_scc = i;
        }
    }
    if (num_scc == 0) {
        printf("NIE\n");
    }
    else if (num_scc > 1) {
        printf("0\n\n");
    }
    else {
        int N = scc_cnt[check_scc];

        int relabel = 0;
        for (int u = 0; u < n; u++) {
            if (scc[u] == check_scc) {
                relabel_to[u] = relabel;
                relabel_from[relabel] = u;
                relabel++;
            }
        }
        for (int ru = 0; ru < N; ru++) {
            int u = relabel_from[ru];
            for (int i = 0; i < adj[u].size(); i++) {
                int v = adj[u][i];
                if (scc[v] == check_scc) {
                    int rv = relabel_to[v];
                    adj_scc[ru].push_back(rv);
                }
            }
        }

        num_cycles = 0;
        for (int ru = 0; ru < N; ru++) {
            cycle_cnt[ru] = 0;
        }

        for (int ru = 0; ru < N; ru++) {
            marked[ru] = false;
        }
        for (s = 0; s < N; s++) {
            backtrack(s);
            while (!marked_stack.empty()) {
                int u = marked_stack.back();
                marked_stack.pop_back();
                marked[u] = false;
            }
        }

        int res = 0;
        for (int ru = 0; ru < N; ru++) {
            if (cycle_cnt[ru] == num_cycles) {
                res++;
            }
        }
        if (res == 0) {
            printf("0\n\n");
        }
        else {
            printf("%d\n", res);
            int i = 0;
            for (int ru = 0; ru < N; ru++) {
                if (cycle_cnt[ru] == num_cycles) {
                    if (i < res - 1) {
                        printf("%d ", relabel_from[ru] + 1);
                    }
                    else {
                        printf("%d\n", relabel_from[ru] + 1);
                    }
                    i++;
                }
            }
        }
    }

    return 0;
}