1 #include <iostream>
 2 #include <cstdio>
 3 #include <cstdlib>
 4 #include <cstring>
 5 #include <algorithm>
 6 #define ll long long
 7 using namespace std;
 8 const int MAXN = 12000;
 9 const int MAXM = 120000;
10 struct Edge
11 {
12     int to,next;
13 }edge[MAXM];
14 int head[MAXN],tot;
15 int Low[MAXN],DFN[MAXN],Stack[MAXN],Belong[MAXN];
16 int Index,top;
17 int scc;
18 bool Instack[MAXN];
19 int num[MAXN];
20 
21 void addedge(int u,int v)
22 {
23     edge[tot].to = v;edge[tot].next = head[u];head[u] = tot++;
24 }
25 void Tarjan(int u)
26 {
27     int v;
28     Low[u] = DFN[u] = ++Index;
29     Stack[top++] = u;
30     Instack[u] = true;
31     for(int i = head[u]; i != -1; i = edge[i].next)
32     {
33         v = edge[i].to;
34         if( !DFN[v] )
35         {
36             Tarjan(v);
37             if(Low[u] > Low[v]) Low[u] = Low[v];
38         }
39         else if(Instack[v] && Low[u] >  DFN[v])
40             Low[u] = DFN[v];
41     }
42     if(Low[u] == DFN[u])
43     {
44         scc++;
45         do
46         {
47             v = Stack[--top];
48             Instack[v] = false;
49             Belong[v] = scc;
50             num[scc]++;
51         }
52         while(v != u);
53     }
54 }
55 void solve(int N)
56 {
57     memset(DFN,0,sizeof(DFN));
58     memset(Instack,false,sizeof(Instack));
59     memset(num,0,sizeof(num));
60     Index = scc = top = 0;
61     for(int i = 1; i <= N; i++)
62         if( !DFN[i])
63         Tarjan(i);
64 }
65 void init()
66 {
67     tot = 0;
68     memset(head,-1,sizeof(head));
69 }
70 int main(void)
71 {
72     int n,m,a,b;
73     while(scanf("%d %d",&n,&m) ,n != 0 || m != 0)
74     {
75         init();
76         for(int i = 0; i < m; i++)
77         {
78             scanf("%d %d",&a,&b);
79             addedge(a,b);
80         }
81         solve(n);
82         if(scc == 1)
83             printf("Yes\n");
84         else
85             printf("No\n");
86     }
87     return 0;
88 }