1 #include <stdio.h>
 2 #include <algorithm>
 3 #include <string.h>
 4 #include <vector>
 5 #include <stack>
 6 using namespace std;
 7 
 8 int n, m;
 9 vector<int>ve[10005];
10 stack<int>s;
11 int dfn[10005], low[10005];
12 bool instack[10005];
13 int dfn_clock, num;
14 
15 void init()                      //init
16 {
17     int i, j;
18     while(!s.empty())
19     s.pop();
20     for(i=1;i<=n;i++)
21     {
22         ve[i].clear();
23         instack[i]=true;
24     }
25     memset(dfn,0,sizeof(dfn));
26     memset(low,0,sizeof(low));
27 }
28 
29 void tarjan(int u)                    //tarjan
30 {
31     int v;
32     s.push(u);
33     dfn[u]=low[u]=++dfn_clock;
34     for(int i=0;i<ve[u].size();i++)
35     {
36         v=ve[u][i];
37         if(!dfn[v])
38         {
39             tarjan(v);
40             low[u]=min(low[u],low[v]);
41         }
42         else if(instack[v])
43         {
44             low[u]=min(low[u],dfn[v]);
45         }
46     }
47     if(low[u]==dfn[u])
48     {
49         num++;
50         while(1)
51         {
52             v=s.top();
53             s.pop();
54             instack[v]=false;
55             if(v==u) break;
56         }
57     }
58 }
59 main()                       //main
60 {
61     while(scanf("%d %d",&n,&m)==2)
62     {
63         if(n==0&&m==0) break;
64         int i, j, x, y;
65         init();
66         for(i=1;i<=m;i++)
67         {
68             scanf("%d %d",&x,&y);
69             ve[x].push_back(y);
70         }
71         num=dfn_clock=0;
72         for(i=1;i<=n;i++)
73         if(!dfn[i])
74         tarjan(i);
75         if(num>1) printf("No\n");
76         else printf("Yes\n");
77     }
78 }