#!/usr/bin/env python
#coding:utf8


def read():
    N, K = map(int, raw_input().split())
    vList = map(int, raw_input().split())
    return K, vList


def calcCost(subList):
    ret = 0
    for i in range(len(subList)):
        for j in range(len(subList) - i - 1):
            if subList[j] > subList[j + 1]:
                subList[j], subList[j + 1] = subList[j + 1], subList[j]
                ret += 1
    return ret


def work((K, vList)):
    
    for i in range(K - 1):
        if i + K >= len(vList) and i + 1 < len(vList) and vList[i] > vList[i + 1]:
            print -1
            return

    ans = 0
    for i in range(min(K, len(vList) - K)):
        ans += calcCost(vList[i::K])
        vList[i::K] = sorted(vList[i::K])
    
    
    print ans if vList == sorted(vList) else -1


if __name__ == "__main__":
    work(read())