#include <bits/stdc++.h>
using namespace std;
const long long N = 505;
long long n, m, pr[N], deg[N];
bool d[N][N], used[N];
vector<long long> a;
void dfs(long long u) {
  used[u] = true;
  for (long long i = 1; i <= n; ++i)
    if (!used[i] && d[u][i])
      if (--deg[i] <= 0) dfs(i);
}
signed main() {
  ios_base::sync_with_stdio(false);
  cin >> n >> m;
  while (m--) {
    long long x, y;
    cin >> x >> y;
    d[x][y] = true;
    pr[y]++;
  }
  for (long long i = 1; i <= n; i++) {
    for (long long j = 1; j <= n; j++) deg[j] = pr[j], used[j] = false;
    deg[i] -= 1;
    for (long long j = 1; j <= n; j++)
      if (!used[j] && deg[j] <= 0) dfs(j);
    bool check = false;
    for (long long j = 1; j <= n; j++)
      if (!used[j]) {
        check = true;
        break;
      }
    if (!check) {
      cout << "yes";
      return 0;
    }
  }
  cout << "no";
}
