N, K = map(int, input().split())
a = list(map(int, input().split()))
b = a.copy()
b.sort()
if a == b:
    print(0)
    exit()
cnt = 0
for _ in range(N - K):
    for i in range(N - K):
        if a[i] > a[i + K]:
            a[i], a[i + K] = a[i + K], a[i]
            cnt += 1
            if a == b:
                print(cnt)
                exit()
print(-1)