K

#include <iostream>
#include <vector>

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

    bool f = false;

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

    int j,k;
    for (int x = 0; x < b; x++) {
        std::cin>>j>>k;
        for (int i = 0; i < pairs.size(); i++) {
            if ((pairs[i].first == j+1 && pairs[i].second == k+1)) {
                f = true;
                break;
            }
        }

        pairs.push_back(std::pair<int,int>(j+1,k+1));
    }

    if (f) {
        std::cout<<"YES"<<std::endl;
    } else {
        std::cout<<"NO"<<std::endl;
    }


    return 0;
}