#include <bits/stdc++.h>
using namespace std;

const int N = 110, M = N << 1;
int in[N]; // 入度
// 链式前向星
int e[M], h[N], idx, w[M], ne[M];
void add(int a, int b, int c = 0) {
    e[idx] = b, ne[idx] = h[a], w[idx] = c, h[a] = idx++;
}

bool solve(int n) {
    queue<int> q;
    for (int i = 0; i < n; i++)
        if (in[i] == 0) q.push(i);

    int cnt = 0;
    while (q.size()) {
        int u = q.front();
        q.pop();

        ++cnt;
        for (int i = h[u]; ~i; i = ne[i]) {
            int j = e[i];
            in[j]--;
            if (in[j] == 0) q.push(j);
        }
    }
    return cnt == n;
}

int main() {
    // 加快读入
    ios::sync_with_stdio(false), cin.tie(0);
    int n, m;
    while (cin >> n >> m && n) {
        // 多组测试数据，一定要注意两个初始语句！
        memset(h, -1, sizeof h);
        idx = 0;
        memset(in, 0, sizeof in);

        while (m--) {
            int u, v;
            cin >> u >> v;
            in[v]++;
            add(u, v); // 建正图
        }
        bool ans = solve(n);
        puts(ans ? "YES" : "NO");
    }
    return 0;
}