Q

#include <iostream>
#include <vector>

using namespace std;

int main() {
    int a, b;
    std::cin>>a>>b;
    int matrix[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++) {
            matrix[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) {
                    matrix[x][y] = 1;
                    matrix[y][x] = 1;
                }
            }
        }

    std::vector<int> ans;

    int g = 0;
    for (int x = 0; x < a; x++) {
        for (int y = 0; y < a; y++) {
            if (matrix[x][y]) {
                g++;
            }
        }
        ans.push_back(g);
        g=0;
    }

    bool v = false;

    for (int x = 1; x < a; x++) {
       if (ans[x] != ans[x-1]) {
           v = true;
       }
    }

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