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 " );
        
    }
}