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

ans = 0
for i in range(K):
    l = A[i::K]
    # バブルソート
    for j in range(len(l)):
        for k in reversed(range(j + 1, len(l))):
            if l[k - 1] > l[k]:
                l[k - 1], l[k] = l[k], l[k - 1]
                ans += 1
    A[i::K] = l
print(ans if A == sorted(A) else -1)