# -*- coding: utf-8 -*-

N,K = map(int, input().split())
a = list(map(int, input().split()))

ans = 0
for i in range(0, N-K, 1):
    for j in range(N-1-K, i-1, -1):
        if a[j] > a[j+K]:
            temp = a[j]
            a[j] = a[j+K]
            a[j+K] = temp
            ans += 1

is_sorted = True
for i in range(N-1):
    if a[i] > a[i+1]:
        is_sorted = False
        break

print(ans) if is_sorted else print(-1)