from itertools import permutations
import sys

sys.setrecursionlimit(10 ** 6)
from bisect import *
from collections import *
from heapq import *

def II(): return int(sys.stdin.readline())
def MI(): return map(int, sys.stdin.readline().split())
def LI(): return list(map(int, sys.stdin.readline().split()))
def SI(): return sys.stdin.readline()[:-1]
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
int1 = lambda x: int(x) - 1
def MI1(): return map(int1, sys.stdin.readline().split())
def LI1(): return list(map(int1, sys.stdin.readline().split()))
p2D = lambda x: print(*x, sep="\n")
dij = [(1, 0), (0, 1), (-1, 0), (0, -1)]

def main():
    n,k=MI()
    aa=LI()
    bb=[[] for _ in range(k)]
    for i,a in enumerate(aa):bb[i%k].append(a)
    ans=0
    for row in bb:
        for i,a in enumerate(row):
            for pa in aa[:i]:
                if pa>a:ans+=1
    for i in range(k):bb[i].sort()
    pa=0
    for j in range(len(bb[0])):
        for i in range(k):
            if j==len(bb[i]):
                print(ans)
                exit()
            if bb[i][j]<pa:
                print(-1)
                exit()
            pa=bb[i][j]
    print(ans)

main()