N, K = map(int, input().split())
a = list(map(int, input().split()))
b = sorted(a)
s = 0
for i in reversed(range(K, N)):
    for j in range(i-K+1):
        if a[j] > a[j+K]:
            a[j], a[j+K] = a[j+K], a[j]
            s += 1
print(s if a == b else -1)