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

for start in range(K):
    for end in reversed(range(start, N - K, K)):
        for a in range(start, end + 1, K):
            b = a + K
            if A[a] > A[b]:
                A[a], A[b] = A[b], A[a]
                ans += 1
for i in range(1, N):
    if A[i] < A[i - 1]:
        print(-1)
        sys.exit()
print(ans)