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#include <stdio.h>
#include <set>
#include <vector>

using namespace std;

struct Node {
    vector<int> next;
    bool visited;
    int number;
};

int dfs(Node *graph, int start, int number, int removeNode = -1) {
    Node *node = graph + start;

    node->visited = true;
    for(vector<int>::iterator it = node->next.begin(); it != node->next.end(); ++it) {
        if(*it == removeNode) {
            continue;
        }
        if(!graph[*it].visited) {
            number = dfs(graph, *it, number, removeNode);
        }
    }
    node->number = number--;

    return number;
}

void clearVisited(Node *graph, int size) {
    for(int i = 0; i < size; ++i) {
        graph[i].visited = false;
    }
}

bool isAcyclic(Node *graph, int size, int removeNode = -1) {
    clearVisited(graph, size);

    int number = size;
    for(int i = 0; i < size; ++i) {
        if(!graph[i].visited) {
            number = dfs(graph, i, number, removeNode);
        }
    }

    for(int i = 0; i < size; ++i) {
        if(i == removeNode) {
            continue;
        }
        Node *node = graph + i;
        for(vector<int>::iterator it = node->next.begin(); it != node->next.end(); ++it) {
            if(*it == removeNode) {
                continue;
            }
            if(node->number > graph[*it].number) {
                return false;
            }
        }
    }

    return true;
}

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

    if(isAcyclic(graph, n)) {
        printf("NIE\n");
    } else {
        vector<int> result;
        for(int i = 0; i < n; ++i) {
            if(isAcyclic(graph, n, i)) {
                result.push_back(i);
            }
        }
        printf("%d\n", result.size());
        for(vector<int>::iterator it = result.begin(); it != result.end(); ++it) {
            printf("%d ", *it + 1);
        }
        printf("\n");
    }

    return 0;
}