#!/usr/bin/env python3
# %%
import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines


# %%
N, K = map(int, readline().split())
m = map(int, read().split())
AB = tuple(zip(m, m))

# %%
answer = K - 1 if K <= N else - 1
print(answer)