# -*- coding: utf-8 -*-
import sys,copy,math,heapq,itertools as it,fractions,re,bisect,collections as coll

N, K = map(int, raw_input().split())
a = map(int, raw_input().split())

i = ans = 0
while i < N - K:
    if a[i] > a[i + K]:
        a[i], a[i + K] = a[i + K], a[i]
        ans += 1
        i = 0
    else:
        i += 1
if min(a[i + 1] - a[i] for i in xrange(N - 1)) >= 1:
    print ans
else:
    print -1