1 #include<iostream>
 2 #include<cstring>
 3 #include<cstdio>
 4 #include<cstdlib>
 5 #include<cmath>
 6 #include<algorithm>
 7 using namespace std;
 8 typedef long long ll;
 9 #define MAXN 10010
10 #define MAXM 100010
11 struct edge{
12     int to,nxt;
13 }e[MAXM];
14
15 int head[MAXN],st[MAXN],dfn[MAXN],lowest[MAXN],belong[MAXN];
16 bool inst[MAXN];
17 int n,m,scnt,top,tot;
18 void init(){
19     memset(head,-1,sizeof head);
20     memset(dfn, 0, sizeof dfn);
21     scnt=top=tot=0;
22 }
23
24 void add_edge(int u, int v){
25     e[tot].to=v;
26     e[tot].nxt=head[u];
27     head[u]=tot++;
28 }
29
30 void Tarjan(int u){
31     dfn[u]=lowest[u]=++tot;
32     inst[u]=1;
33     st[top++]=u;
34     for(int i=head[u];i!=-1;i=e[i].nxt){
35         int v=e[i].to;
36         if(!dfn[v]){
37             Tarjan(v);
38             lowest[u]=min(lowest[u],lowest[v]);
39         }
40         else if(inst[v]){
41             lowest[u]=min(lowest[u],dfn[v]);//也可用lowest
42         }
43     }
44     if(dfn[u]==lowest[u]){
45         scnt++;
46         int t;
47         do{
48             t=st[--top];
49             inst[t]=false;
50             belong[t]=scnt;
51         }while(t!=u);
52     }
53 }
54
55 inline int read(){
56     char k=0;char ls;ls=getchar();for(;ls<'0'||ls>'9';k=ls,ls=getchar());
57     int x=0;for(;ls>='0'&&ls<='9';ls=getchar())x=(x<<3)+(x<<1)+ls-'0';
58     if(k=='-')x=0-x;return x;
59 }
60
61
62 void solve(){
63     for(int i=1;i<=n;i++)
64         if(!dfn[i])
65             Tarjan(i);
66 }
67
68 int main(){
69     int a,b;
70     while(scanf("%d%d",&n,&m)&&(n||m)){
71         init();
72         while(m--){
73             a=read(),b=read();
74             add_edge(a,b);
75         }
76         solve();
77         if(scnt==1) printf("Yes\n");
78         else printf("No\n");
79     }
80     return 0;
81 }