tjut 1269

/*2012-12-06 20:16:08   Accepted    1269    46MS    1484K   1339 B  C++*/
#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" );
}
}