MOD = 10 ** 9 + 7
INF = 10 ** 10
import sys
sys.setrecursionlimit(100000000)
dy = (-1,0,1,0)
dx = (0,1,0,-1)

def main():
    n,k = map(int,input().split())
    for _ in range(n - 1):
        a,b = map(int,input().split())
    if n < k:
        print(-1)
    else:
        print(k - 1)
if __name__ =='__main__':
    main()