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

int n, m, x[200200], y[200200], chk[200200], chk2[200200];
vector<int> v[200200];
set<array<int, 2>> s;

array<int, 3> dfs(int u) {
    chk2[u] = 1;
    array<int, 3> re = {1, (int)v[u].size(), chk[u]};
    for(auto x : re) if(!chk2[u]) {
        auto [A, B, C] = dfs(x);
        re[0] += A, re[1] += B, re[2] |= C;
    }
    return re;
}

int main() {
    cin.tie(0)->sync_with_stdio(0);
    cin >> n >> m;
    for(int i=1;i<=m;i++) cin >> x[i] >> y[i];
    for(int i=1;i<=m;i++) s.insert({x[i], y[i]});
    for(int i=1;i<=m;i++) {
        if(!s.count({y[i], x[i]})) s.erase({x[i], y[i]}), chk[y[i]] = 1;
        else v[x[i]].push_back(y[i]);
    }

    for(int i=1;i<=n;i++) if(!chk[i] && !chk2[i]) {
        auto [x, y, flag] = dfs(i);
        if(!flag && x == y/2+1) return cout << "NO", 0;
    }
    cout << "YES";
}