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

ans = 0

for s in range(K):
    L = A[s:N:K]
    for i in range(len(L)):
        for b in L[i+1:]:
            ans += L[i] > b
    A[s:N:K] = sorted(L)

print(ans if A == sorted(A) else -1)