hdu3342

/*
 * File:   hdu3342.cpp，拓扑排序
 * Author: chenjiang
 *
 * Created on 2010年5月10日, 下午1:49
 */

#include <stdlib.h>
#include <iostream>
#include <string.h>
#include <algorithm>
#include <queue>
#include <vector>
using namespace std;
#define _max 105
int n, m;
vector<int>v[_max];
int indegree[_max];

bool top_sort() {
    int i, j;
    queue<int>Q;
    for (i = 0; i < n; i++) {
        if (indegree[i] == 0)Q.push(i);
    }
    int ans = 0;
    while (!Q.empty()) {
        int temp = Q.front();
        Q.pop();
        ans++;
        for (i = 0; i < v[temp].size(); i++) {
            indegree[v[temp][i]]--;
            if (indegree[v[temp][i]] == 0)Q.push(v[temp][i]);
        }
    }
    if (ans < n)return 0;
    else return 1;
}

/*
 *
 */
int main(int argc, char** argv) {

    int i, j;
    while (cin >> n >> m) {
        if (n == 0 && m == 0)break;
        for (i = 0; i < n; i++) {
            v[i].clear();
            indegree[i] = 0;
        }
        for (i = 1; i <= m; i++) {
            int a, b;
            cin >> a >> b;
            v[a].push_back(b);
            indegree[b]++;
        }
        bool flag = top_sort();
        if (flag)cout << "YES" << endl;
        else cout << "NO" << endl;
    }
    return (EXIT_SUCCESS);
}