S

#include <iostream>
#include <vector>

using namespace std;

int main() {
    int n, m;
    cin>>n>>m;

    std::pair<int, int> a[m];

    for (int i = 0; i < m; i++) {
        cin>>a[i].first>>a[i].second;
        a[i].first--;
        a[i].second--;
    }

    vector<int> h;

    bool r = false;

    int matrix[n][n];

    for (int x = 0; x < n; x++) {
        for (int y = 0; y < n; y++) {
            for (int i = 0; i < m; i++) {
                if (a[i].first == x && a[i].second == y) {
                    r = true;
                    matrix[x][y] = 1;
                    break;
                }
            }

            if (!r) matrix[x][y] = 0;
            r = false;
        }
    }

    bool g = false;
    for (int x = 0; x < n; x++) {
        for (int y = 0; y < n; y++) {
            if (matrix[x][y] == 0 && x != y && matrix[y][x] == 0) {
                g = true;
            }
        }
    }

    if (!g) {
        cout<<"YES"<<endl;
    } else {
        cout<<"NO"<<endl;
    }
}