#include<iostream>
#define N 1005
using namespace std;
int father[N];
int findFather(int x){
    if(x == father[x]) return x;
    else{
        int tp = findFather(father[x]);
        father[x] = tp;
        return tp;
    }
}
int main(){
    int n,m;
    while(cin>>n>>m){
        int x,y;
        for(int i = 1;i <= n;i++){
            father[i] = i;
        }
        for(int i = 0;i < m; i++){
            cin>>x>>y;
            int faX = findFather(x);
            int faY = findFather(y);
            if(faX != faY){
                father[x] = y;
            }
        }
        int ans = 0;
        for(int j = 1;j <= n; j++){
            if(father[j] == j)
                 ans++;
        }
        if(ans == 1) cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
    return 0;
}