CCC '13 S4 - Who is Taller?

//Ivan Li, Markville secondary school
import java.io.*;
import java.util.*;


public class Main {
  public static void main(String[] args) {
      FastReader sc = new FastReader();

 
      //input int N and M
      //create an array matrix map with dimensions NxN

      //repeat below M times
          //input int bv and ev
          //bv-=1
          //ev-=1
          //set bv and ev as a directional neighbour
          //map[bv][ev]=true;

      //input int p and q
      //p-=1
      //q-=1
      int N = sc.nextInt();
      int M = sc.nextInt();

      ArrayList<Integer> map[] = new ArrayList[N];

      for(int i = 0; i<N; i++){
        map[i] = new ArrayList<Integer>();
      }

      //arraylist is faster when using .get
      //arraylist=LInkedlist

      for(int i=0; i<M; i++){
        int bv = sc.nextInt();  
        int ev = sc.nextInt();
        bv-=1;
        ev-=1;

        map[bv].add(ev);
      }
      int p = sc.nextInt();
      int q = sc.nextInt();
      p-=1;
      q-=1;


      
      //setting p is starting node and q is ending node


      //create a linkedlist called list
      //add p to list

      LinkedList<Integer> list = new LinkedList<Integer>();
      list.add(p);

      //create mincost array with dimension N
      //fill mincost array with Integer_MAXVALUE
      //set mincost[p] to 0

      int mincost[] = new int[N];
      
      Arrays.fill(mincost, Integer.MAX_VALUE);
      mincost[p] = 0;
        

      //while loop with condition List.isEmpty()
      //get next node inside of list using .poll and store into variable next
      //use for loop to find every neighbour of next and check if the cost to traverse is less than the current mincost of the neighbour
          //if so set mincost of neighbour to cost to traverse and add neighbour to list

      while (!list.isEmpty()){
        int next = list.poll();
        
        for(int i = 0; i<map[next].size(); i++){
          int neighbour = map[next].get(i);
          if(mincost[next]+1<mincost[neighbour]){
            list.add(neighbour);
            mincost[neighbour]=mincost[next]+1;
          }
        }
      }
                  
      //if mincost[q]!=Integer.Integer_MAXVALUE then output "yes" and return

      if(mincost[q]!=Integer.MAX_VALUE){
        System.out.println("yes");
        return;
      }


      
      //setting q is starting node and p is ending node

      list.add(q);


      //fill mincost array with Integer.MAX_VALUE
      //set mincost of q to 0
      Arrays.fill(mincost, Integer.MAX_VALUE);
      mincost[q]=0;
        


      //while loop with condition List.isEmpty()
      //get next node inside of list using .poll and store into variable next
      //use for loop to find every neighbour of next and check if the cost to traverse is less than the current mincost of the neighbour
          //if so set mincost of neighbour to cost to traverse and add neighbour to list
        while (!list.isEmpty()){
        int next = list.poll();
        
        for(int i = 0; i<map[next].size(); i++){
          int neighbour = map[next].get(i);
          if(mincost[next]+1<mincost[neighbour]){
            list.add(neighbour);
            mincost[neighbour]=mincost[next]+1;
          }
        }
      }




      //if mincost[p]!=Integer.Integer_MAXVALUE then output "no" and return
      if(mincost[p]!=Integer.MAX_VALUE){
        System.out.println("no");
        return;
      }
      //output "unknown"
      System.out.println("unknown");

  }

public static class FastReader {
        BufferedReader br;
            StringTokenizer st;
    
            public FastReader() {
                br = new BufferedReader(new InputStreamReader(System.in));
            }
    
            String next() {
                while (st == null || !st.hasMoreElements()) {
                    try {
                        st = new StringTokenizer(br.readLine());
                    } catch (IOException e) {
                        e.printStackTrace();
                    }
                }
                return st.nextToken();
            }
    
            int nextInt() {
                return Integer.parseInt(next());
            }
            long nextLong() {
                return Long.parseLong(next());
            }
    
            double nextDouble() {
                return Double.parseDouble(next());
            }
    
            String nextLine() {
                String str = null;
                try {
                    str = br.readLine();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                return str;
            }
        }

}