#include <iostream>
#include <cstdio>
#include <vector>

using namespace std;

typedef long long ll;
typedef pair<int, int> pii;

#define rep(i, n) for(int i = 0; i < n; ++i)
#define ri(a) scanf("%d", &a)
#define rii(a, b) scanf("%d%d", &a, &b)
#define riii(a, b, c) scanf("%d%d%d", &a, &b, &c)
#define Ttimes int T; ri(T); for(int ks = 1; ks <= T; ++ks)

const int dx[] = {1, 0, -1, 0};
const int dy[] = {0, 1, 0, -1};

vector<int> g[110];
int in[110];

int main() {
    int n, m, a, b;
    while(rii(n, m) == 2) {
        rep(i, n + 1) {
            g[i].clear();
            in[i] = 0;
        }
        rep(i, m) {
            rii(a, b);
            g[a].push_back(b);
            in[b]++;
        }
        bool fg = false;
        rep(i, n) {
            int x = 0;
            for(int i = 1; i <= n; ++i)
                if(in[i] == 0) {
                    x = i;
                    break;
                }
            if(x == 0) {
                fg = true;
                break;
            }
            rep(i, g[x].size())
                in[g[x][i]]--;
            in[x] = -1;
        }
        puts(fg ? "NO" : "YES");
    }
    return 0;
}