迷宫城堡//强连通分支Tarjan

#include <iostream>
#include <vector>
using namespace std;
const int size = 1e4 + 1;

vector<int> graph[size];//存图
int dfn[size];
int t = 0;
int low[size];
int stack[size];
int pos = 0;//用于栈的索引
bool onstack[size];//是否在栈
bool marked[size];
int ans = 0;//连通分支数

void init(int N)//初始化
{
    t = 0;
    pos = 0;
    ans = 0;
    for(int i = 1; i <= N; i++)
    {
        graph[i].clear();
        onstack[i] = false;
        marked[i] = false;
    }
}

void Tarjan(int x)
{
    marked[x] = true;
    low[x] = dfn[x] = t++;
    onstack[x] = true;
    stack[pos++] = x;
    for(int i = 0; i < graph[x].size(); i++)
    {
        if(!marked[graph[x][i]])//未标记
        {
            Tarjan(graph[x][i]);
            if(low[graph[x][i]] < low[x])
                low[x] = low[graph[x][i]];//！！！
        }
        //在栈内
        else if(onstack[graph[x][i]] && dfn[graph[x][i]] < low[x])
            low[x] = dfn[graph[x][i]];//！！！
    }
    if(dfn[x] == low[x])//强连通分支的根
    {
        ans++;
        while(stack[--pos] != x)//出栈
        {
            int c = stack[pos];
            onstack[c] = false;
        }
        onstack[x] = false;
    }
}

int main()
{
    int N=1, M;
    int a, b;
    while(cin >> N >> M && N != 0 || M != 0)
    {
        //初始化
        init(N);
        //输入
        for(int i = 0; i < M; i++)
        {
            cin >> a >> b;
             graph[a].push_back(b);    
        }

        for(int i = 1; i <= N; i++)
        {
            if(!marked[i])
                Tarjan(1);
        }

        if(ans == 1)
            printf("Yes\n");
        else 
            printf("No\n");
    }
    return 0;
}