R

#include <iostream>
#include <vector>

using namespace std;

int main() {
    int a, b;
    std::cin>>a>>b;
    int map[a][a];

    std::vector<std::pair<int, int>> pairs;

    int j,k;
    for (int x = 0; x < b; x++) {
        std::cin>>j>>k;
        pairs.push_back(std::pair<int,int>(j-1,k-1));
    }

    for (int x = 0; x < a; x++) {
        for (int y = 0; y < a; y++) {
            map[x][y] = 0;
        }
    }

    for (int x = 0; x < a; x++)
        for (int y = 0; y < a; y++) {
            for (int l = 0; l < pairs.size(); l++) {
                if (pairs[l].first == x && pairs[l].second == y) {
                    map[x][y] = 1;
                    map[y][x] = 1;
                }
            }
        }

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

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