#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
using namespace std;
const int maxn = 10000 + 10;
vector<int>g[maxn];
bool inStack[maxn];
int Stack[maxn],top,Tar;
int dfn[maxn],low[maxn];
int n,m,Time;
void init(){
    memset(inStack,-1,sizeof inStack);
    memset(dfn,-1,sizeof dfn);
    top = Tar = Time = 0;
    for (int i = 0; i <= n; ++i)g[i].clear();
}
void dfs(int k){
    dfn[k] = low[k] = ++Time;
    Stack[++top] = k;
    inStack[k] = 1;
    int len = g[k].size();
    for (int i = 0; i < len; ++i){
        int v = g[k][i];
        if (dfn[v] == -1){
            dfs(v);
            low[k] = min(low[k],low[v]);
        }
        else if (inStack[v]){
            low[k] = min(low[k],dfn[v]);
        }
    }
    if (dfn[k] == low[k]){
        ++Tar;
        int cur = Stack[top];
        while(cur != k){
            inStack[cur] = 0;
            cur = Stack[--top];
        }
        inStack[cur] = 0;
        --top;
    }
}
int main(){
    while(scanf("%d %d",&n, &m) == 2 && (n || m)){
        init();
        for (int i = 0; i < m; ++i){
            int u,v;
            scanf("%d %d",&u, &v);
            g[u].push_back(v);
        }
        for (int i = 1; i <= n; ++i){
            if (dfn[i] == -1) dfs(i);
        }
        if (Tar == 1)puts("Yes");
        else puts("No");
    }

    return 0;
}