package weblab;

import java.io.*;
import java.util.*;

class Node {

  List<Node> outgoingEdges;

  boolean marked;

  public Node() {
    this.outgoingEdges = new ArrayList<>();
    this.marked = false;
  }
}

class Solution {

  // Implement the solve method to return the answer to the problem posed by the inputstream.
  public static String run(InputStream in) {
    return new Solution().solve(in);
  }

  public String solve(InputStream in) {

    Scanner scanner = new Scanner(in);
    int n = scanner.nextInt();
    int m = scanner.nextInt();
    int s = scanner.nextInt();
    int t = scanner.nextInt();

    Node[] nodes = new Node[n];

    for(int i = 0; i < n; i++) {
      nodes[i] = new Node();
    }
    for(int j = 0; j < m; j++) {
      int start = scanner.nextInt();
      int end = scanner.nextInt();
      scanner.nextInt();
      nodes[start-1].outgoingEdges.add(nodes[end-1]);
    }
    scanner.close();
    Node start = nodes[s-1];
    Node end = nodes[t-1];
    if(start.equals(end)) return "yes";

    //where your code starts

    Stack<Node> stack = new Stack<Node>();
    stack.push(start);
    while (!stack.empty()){
      Node currNode = stack.pop();
      for (Node node : currNode.outgoingEdges){
        //System.out.println("Node " + currNode.id + " checking edge " + node.id);
        if (node.equals(end)){
          return "yes";
        }
        if(!node.marked)
          stack.push(node);
          node.marked = true;

      }
      //System.out.println("On top of the stack : " + stack.peek().id);
      //System.out.println("Stack empty? " + stack.empty());
    }
    return "no";

    //where your code ends

  }
  //   Stack<Node> stack = new Stack<Node>();
  //   stack.push(start);
  //   start.marked = true;
  //   while(stack.size() != 0){
  //     Node sblh = stack.pop();
      // for(Node a : sblh.outgoingEdges) {
      //   if(a.equals(end)) return "yes";
      //   if(!a.marked) stack.push(a);
      //   a.marked = true;
      // }
  //   }

  //   return "no";
  // }
}