# coding: utf-8
# Your code here!

import sys
readline = sys.stdin.readline
read = sys.stdin.read

#n,*a = [int(i) for i in read().split()]
#a = [int(i) for i in readline().split()]
#n = int(input())


n,k,*mm = [int(i) for i in read().split()]

if k > n:
    print(-1)
    exit()

else:
    print(k-1)