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#include<bits/stdc++.h>#define INIT(a,b) memset(a,b,sizeof(a))#define LL long longusing namespace std;const int inf=0x3f3f3f3f;const int maxn=1e4+7;const int maxm=1e5+7;const int mod=1e9+7;struct Edge{    int from,to,next;}edge[maxm];int Begin[maxn],dfn[maxn],low[maxn],vis[maxn],sta[maxn],color[maxn];int n,m,tot,top,dfsNum,colNum;

void init(){    INIT(Begin,-1);INIT(vis,0);    INIT(low,0);INIT(dfn,0);    INIT(color,0);    top=-1,tot=0,dfsNum=0,colNum=0;}void add(int x,int y){    edge[tot]=(Edge){x,y,Begin[x]};    Begin[x]=tot++;}void Tarjan(int x){    dfn[x]=low[x]=++dfsNum; //标记时间戳    vis[x]=1;    sta[++top]=x; //入栈    for(int i=Begin[x];~i;i=edge[i].next){        int ne=edge[i].to;        if(!dfn[ne]){            Tarjan(ne);            low[x]=min(low[x],low[ne]);        }        else if(vis[ne]) low[x]=min(low[x],dfn[ne]); //找到环    }    if(low[x]==dfn[x]){        vis[x]=0;        color[x]=++colNum;        //出栈并染色        while(sta[top]!=x){            vis[sta[top]]=0;            color[sta[top--]]=colNum;        }        top--;//弹出x    }}int main(){    int x,y;    while(~scanf("%d%d",&n,&m)&&(n||m)){        init();        while(m--){            scanf("%d%d",&x,&y);            add(x,y);        }        for(int i=1;i<=n;i++)            if(!dfn[i]) Tarjan(i);        if(colNum==1) printf("Yes\n");        else printf("No\n");

    }    return 0;}