void BFS()
{
   int u,v,i,l;
   while(!Q.empty()) Q.pop();
   memset(check,false,sizeof(check));
   memset(p,-1,sizeof(p));
   for(i=source; i<=sink; i++) d[i]=INF;
   d[1]=0;
   queue<int>Q;
   Q.push(1);check[1]=true;
   while(!Q.empty())
   {
      u=Q.front();
      Q.pop();
      //printf("u:%d\n",u);
      check[u]=false;
      l=adj[u].size();
      for(i=0; i<l; i++)
      {
         v=adj[u][i];
         if(cap[u][v]-flow[u][v]>0 && d[v]>d[u]+cost[u][v])
         {
            d[v]=d[u]+cost[u][v];
            p[v]=u;
            if(!check[v])
            {
               check[v]=true;
               Q.push(v);
            }
         }
      }
   }

}