#include <iostream>
#include <set>
#include <vector>
#include <deque>

#define se second
#define fi first
#define mp make_pair
#define pb push_back

using namespace std;

int n, m;
vector<vector<int>> g;
vector<bool> used;
bool cycle;

void dfs(int h) {
    used[h] = true;
    for (auto e : g[h]) {
        if (used[e]) {
            cycle = true;
            return;
        }
        if (!used[e]) dfs(e);
    }
}

int main() {
    cin >> n >> m;
    vector<bool> is_root(n, true);
    for (int i = 0; i < m; i++) {
        int x, y;
        cin >> x >> y;
        x--, y--;
        g[x].pb(y);
        is_root[y] = false;
    }
    used.assign(n, false);
    cycle = false;
    for (int i = 0; i < n; i++) {
        if (!used[i]) dfs(i);
    }
    if (cycle) {
        cout << "No";
        return 0;
    }
    for (int i = 0; i < n; i++) {
        if (is_root[i])
    }
    return 0;
}