zoj2740

 

#include<cstdio>
#include<iostream>
#include<vector>
using namespace std;
vector<int>adj[1002];
int n,m;
bool f[1002];
void dfs(int v){
    f[v]=1;
    for(int i=0;i<adj[v].size();i++){
        if(f[adj[v][i]]==1)
            continue;
        dfs(adj[v][i]);
    }
}
int main()
{
    int i,j;
    while(scanf("%d %d",&n,&m),n||m){
        if(m!=n-1){
            while(m--)
                scanf("%d%d",&i,&j);
            printf("No\n");
            continue;
        }
        int a,b;
        for(i=1;i<=n;i++)
            adj[i].clear();
        while(m--){
            scanf("%d%d",&a,&b);
            adj[a].push_back(b);
            adj[b].push_back(a);
        }
        memset(f,0,sizeof(f));
        dfs(1);
        for(i=1;i<=n;i++){
            if(f[i]==0)
                break;
        }
        if(i>n)    printf("Yes\n");
        else    printf("No\n");
        
    }
}