N, K = map(int, input().split())
a = list(map(int, input().split()))
b = list(a)
b.sort()

ans = 0
for k in range(K):
    for j in range(k, N, K):
        for i in range(k, j, K):
            if a[i] > a[j]:
                a[i], a[j] = a[j], a[i]
                ans += 1

if a == b:
    print(ans)
else:
    print(-1)