n, k = map(int,input().split())
a = list(map(int, input().split()))
b = sorted(a)

c = 0
for i in range(n):
    for j in range(n-i-k):
        if a[j] > a[j+k]:
            a[j+k], a[j] = a[j], a[j+k]
            c += 1

print(c if a == b else -1)