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

#define rep(i, a, b) for(int i = a; i < (b); ++i)
#define all(x) begin(x), end(x)
#define sz(x) (int)(x).size()
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<int> vi;

int main() {
	cin.tie(0)->sync_with_stdio(0);
	cin.exceptions(cin.failbit);

  int n, m;
  cin >> n >> m;
  vi p(n), s(n, 1);
  iota(all(p), 0);

  auto find = [&](auto self, int v)->int {
    if (v == p[v]) return v;
    return p[v] = self(self, p[v]);
  };

  auto merge = [&](int u, int v)->bool {
    u = find(find, u);
    v = find(find, v);
    if (u == v) return false;
    if (s[u] < s[v]) swap(u, v);
    s[u] += s[v];
    p[v] = u;
    return true;
  };

  auto norm = [&](pii&& p)->pii {
    if (p.first > p.second) return {p.second, p.first};
    return p;
  };

  set<pii> edgeset;
  set<pii> undir;
  vector<pii> edges;
  vi indeg(n);

  rep(i, 0, m) {
    int u, v;
    cin >> u >> v;
    u--,v--;
    auto e = norm(make_pair(u, v));
    if (edgeset.count(e)) {
      undir.insert(e);
    } else edgeset.insert(e);
    edges.push_back({u, v});
  }

  for (auto& e : edges) {
    if (undir.count(norm(std::forward<pii>(e))))
      continue;
    indeg[e.second]++;
  }

  vector<int> is_ok(n);
  for (auto& e : undir) {
    if (!merge(e.first, e.second)) {
      is_ok[e.first] = is_ok[e.second] = 1;
    }
    if (indeg[e.first] > 0 || indeg[e.second] > 0) {
      is_ok[e.first] = is_ok[e.second] = 1;
    }
  }

  rep(i, 0, n) is_ok[find(find, i)] |= is_ok[i];

  bool ok = true;
  rep(i, 0, n) {
    if (find(find, i) == i && indeg[i] == 0) {
      ok &= is_ok[i];
    }
  }
  cout << (ok?"YES":"NO") << '\n';
}