from collections import *
from itertools import *
from functools import *
from heapq import *
import sys,math
input = sys.stdin.readline


def mergecount(A):
    cnt = 0
    n = len(A)
    if n>1:
        A1 = A[:n>>1]
        A2 = A[n>>1:]
        cnt += mergecount(A1)
        cnt += mergecount(A2)
        i1=0
        i2=0
        for i in range(n):
            if i2 == len(A2):
                A[i] = A1[i1]
                i1 += 1
            elif i1 == len(A1):
                A[i] = A2[i2]
                i2 += 1
            elif A1[i1] <= A2[i2]:
                A[i] = A1[i1]
                i1 += 1
            else:
                A[i] = A2[i2]
                i2 += 1
                cnt += n//2 - i1
    return cnt


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

B = [[] for _ in range(K)]
C = [[] for _ in range(K)]

for i in range(N):
    idx = i%K
    B[idx].append(A[i])
    C[idx].append(A[i])

for i in range(K):
    C[i].sort()
T = [0]*N
for i in range(N):
    p,r = divmod(i,K)
    T[i]=C[r][p]
if any(T[i]>=T[i+1] for i in range(N-1)):
    print(-1)
    exit()
ans = 0
for b in B:
    ans += mergecount(b)
print(ans)