Contest1263/A.cpp

#include <bits/stdc++.h>
using namespace std;
const int MAXN = 1001;
int F[MAXN];
int father(int x){
    return x == F[x] ? F[x] : F[x] = father(F[x]);
}
void merge(int x, int y){
    int rx = father(x);
    int ry = father(y);
    if(rx != ry){
        F[rx] = ry;
    }
}
int main(){
    int n, m;
    while(cin >> n >> m){
        if(n == 0)break;
        for(int i = 1; i <= n; i++){
            F[i] = i;
        }
        for(int i = 1; i <= m; i++){
            int x, y;
            cin >> x >> y;
            merge(x, y);
        }
        int cnt = 0;
        for(int i = 1; i <= n; i++){
            if(F[i] == i)cnt++;
        }
        if(cnt > 1){
            cout << "NO" << endl;
        }else{
            cout << "YES" << endl;
        }
    }
}