#region Header
#!/usr/bin/env python3
# from typing import *

import sys
import io
import math
import collections
import decimal
import itertools
from queue import PriorityQueue
import bisect
import heapq

def input():
    return sys.stdin.readline()[:-1]

sys.setrecursionlimit(1000000)
#endregion

# _INPUT = """# paste here...
# """
# sys.stdin = io.StringIO(_INPUT)


def main():
    N, K = map(int, input().split())
    a = [None for _ in range(N - 1)]
    b = [None for _ in range(N - 1)]
    for i in range(N - 1):
        a[i], b[i] = map(int, input().split())
    
    if N < K:
        print(-1)
    else:
        print(K-1)

if __name__ == '__main__':
    main()