import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

import numpy as np

N,K = map(int,readline().split())
A = np.array(read().split(),np.int64)

def Inversion(A):
    return sum(np.count_nonzero(A[:i] > x) for i,x in enumerate(A))

x = sum(Inversion(A[i::K]) for i in range(K))

B = np.empty_like(A)
for i in range(K):
    B[i::K] = np.msort(A[i::K])

if np.any(B[:-1] > B[1:]):
    print(-1)
else:
    print(x)