#include <stack>

#include <vector>

#include <cstdio>

#include <cstring>

#include <iostream>

using namespace std;

const int MAX_N =  + ;

vector<int> G[MAX_N];

stack<int> S;

int pre[MAX_N], lowlink[MAX_N], scc_no[MAX_N];

int dfs_clock, scc_cnt;

inline int Min(int a, int b) { return a < b ? a : b; }

void Dfs(int u)

{

    pre[u] = lowlink[u] = ++dfs_clock;

    S.push(u);

    for (int i = ; i < G[u].size(); ++i)

    {

        int v = G[u][i];

        if (!pre[v])

        {

            Dfs(v);

            lowlink[u] = Min(lowlink[u], lowlink[v]);

        } else if (!scc_no[v])

            lowlink[u] = Min(lowlink[u], lowlink[v]);

    }

    if (lowlink[u] == pre[u])

    {

        scc_cnt++;

        for (;;)

        {

            int x = S.top();

            S.pop();

            scc_no[x] = scc_cnt;

            if (x == u)  break;

        }

    }

}

inline void FindScc(int n)

{

    dfs_clock = scc_cnt = ;

    memset(pre, , sizeof(pre));

    memset(lowlink, , sizeof(lowlink));

    memset(scc_no, , sizeof(scc_no));

    for (int i = ; i <= n; ++i)

    if (!pre[i]) Dfs(i);

}

int main()

{

    int ver_num, road_num;

    while (scanf("%d %d", &ver_num, &road_num) != EOF

        && (ver_num + road_num))

    {

        int ver_1, ver_2;

        for (int i = ; i < MAX_N; ++i)

            G[i].clear( );

        for (int i = ; i < road_num; ++i)

        {

            scanf("%d %d", &ver_1, &ver_2);

            G[ver_1].push_back(ver_2);

        }

        FindScc(ver_num);

        while (!S.empty( ))

            S.pop( );

        if (scc_cnt > )

            printf("No\n");

        else

            printf("Yes\n");

    }

    return ;

}