package main

import (
	"fmt"
)

func main() {
	var N int
	var K int
	fmt.Scanf("%d %d", &N, &K)
	data := make([]int, N)
	for i := 0; i < N; i++ {
		fmt.Scanf("%d", &data[i])
	}
	count := 0
	for i := 0; i < K; i++ {
		for j := i; j < N; j += K {
			for l := i; l < j; l += K {
				if data[j] < data[l] {
					data[j], data[l] = data[l], data[j]
					count += 1
				}
			}
		}
	}
	for t := 0; t < N - 1; t++ {
		if !(data[t] < data[t + 1]) {
			fmt.Println("-1")
			return
		}
	}
	fmt.Println(count)
}