tjut 1269

#include <cstdio>  
#include <iostream>  
#include <vector>  
using namespace std;  
#define maxn 10010  
  
int n;  
vector<int> g[maxn];  
  
int Bcnt; /// 强连通分量的个数；  
int Top; /// 栈顶；  
int Index; /// 时间戳；  
int low[maxn], dfn[maxn];  
int belong[maxn], stack[maxn];  
bool instack[maxn];  
  
void Init_tarjan(){  
    Bcnt= Top= Index= 0;  
    for(int i=1; i<=n; ++i) low[i]= dfn[i]= 0;  
}  
void Tarjan(int u){  
    stack[Top++]= u;  
    instack[u]= 1;  
    low[u]= dfn[u]= ++Index;  
    for(int i=0; i<g[u].size(); ++i){  
        int v= g[u][i];  
        if( !dfn[v] ){  
            Tarjan(v);  
            low[u]= min( low[v], low[u] );  
        }  
        else if( instack[v] )  
            low[u]= min( low[u], dfn[v] );  
    }  
    if( low[u]==dfn[u] ){  
        ++Bcnt;  
        int v;  
        do{  
            v= stack[--Top];  
            instack[v]= 0;  
            belong[v]= Bcnt;  
        }while( u!=v );  
    }  
}  
  
int main()  
{  
    int m;  
    while( cin>>n>>m, n+m ){  
        for(int i=1; i<=n; ++i) g[i].clear();  
        while(m--){  
            int x, y;  
            scanf("%d%d", &x, &y);  
            g[x].push_back(y);  
        }  
        Init_tarjan();  
        for(int i=1; i<=n; ++i){  
            if( !dfn[i] )  
                Tarjan(1);  
        }  
  
        puts( Bcnt==1 ? "Yes" : "No" );  
    }  
}