import java.util.*;

public class Main {
    public static void main(String[] args) throws Exception {
        
        Scanner sc = new Scanner(System.in);
        
        int n = sc.nextInt();
        int k = sc.nextInt();
        
        int[] us = new int[n-1];
        int[] vs = new int[n-1];
        for (int i = 0 , len = n - 1; i < len ; i++){
            us[i] = sc.nextInt();
            vs[i] = sc.nextInt();
        } 
        
        if(k > n){
            System.out.println(-1);
        }else if(k == 1){
            System.out.println(0);
        }else{
            System.out.println(k - 1);
        }
           
    }
}