1 #include <cstdio> 2 #include <cstring> 3 #include <algorithm> 4 using namespace std; 5 const int inf=1<<20; 6  7 int g[200][200]; 8 int main() 9 {10     int n,m;11     while(scanf("%d%d",&n,&m)!=EOF)12     {13         memset(g,0,sizeof(g));14         for(int i=1; i<=n; i++)15         {16             for(int j=1; j<=n; j++)17             {18                 g[i][j]=inf;19             }20         }21         for(int i=0; i<m; i++)22         {23             int a,b;24             scanf("%d%d",&a,&b);25             g[b][a]=1;26         }27         for(int k=1; k<=n; k++)28         {29             for(int i=1; i<=n; i++)30             {31                 for(int j=1; j<=n; j++)32                 {33                     if(i==k||j==k) continue;34                     g[i][j]=min(g[i][j],g[i][k]+g[k][j]);35                 }36             }37         }38         bool flag=true;39         for(int i=1; i<=n; i++)40         {41             if(g[i][i]<inf)42             {43                 flag=false;44                 break;45             }46         }47         if(flag) printf("YES\n");48         else printf("NO\n");49     }50     return 0;51 }