N, K = map(int, input().split())
A = list(map(int, input().split()))
ans = 0

for start in range(K):
    partial = A[start:N:K]
    for a in range(len(partial)):
        for b in range(a + 1, len(partial)):
            if partial[a] > partial[b]:
                ans += 1
    A[start:N:K] = sorted(partial)

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