1 #include<cstdio>
 2 #include<cstring>
 3 #include<vector>
 4 #include<stack>
 5 #include<algorithm>
 6 using namespace std;
 7 
 8 const int maxn=10005;
 9 int lowlink[maxn],dfn[maxn],dfs_clock,m,n;
10 vector<int>G[maxn];
11 stack<int>S;
12 
13 void init()
14 {
15     memset(lowlink,0,sizeof(lowlink));
16     memset(dfn,0,sizeof(dfn));
17     dfs_clock=0;
18     for(int i=1;i<=n;i++)G[i].clear();
19 
20 }
21 
22 int tarjan(int u)
23 {
24     int lowlinku=dfn[u]=++dfs_clock;
25     S.push(u);
26     for(int i=0;i<G[u].size();i++)
27     {
28         int v=G[u][i];
29         if(!dfn[v])
30         {
31             int lowlinkv=tarjan(v);
32             lowlinku=min(lowlinkv,lowlinku);
33         }
34         else
35             lowlinku=min(dfn[v],lowlinku);
36     }
37     lowlink[u]=lowlinku;
38     return lowlinku;
39 }
40 
41 int main()
42 {
43     while(scanf("%d%d",&n,&m),m|n)
44     {
45         init();
46         for(int i=0;i<m;i++)
47         {
48             int u,v;
49             scanf("%d%d",&u,&v);
50             G[u].push_back(v);
51         }
52         for(int i=1;i<=n;i++)
53             if(!dfn[i])
54                 tarjan(i);
55         int j;
56         for(j=1;j<=n-1;j++)
57             if(lowlink[j]!=lowlink[j+1])
58                 break;
59         if(j==n)
60             puts("Yes");
61         else
62             puts("No");
63     }
64     return 0;
65 }