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

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

if a == b:
    print(cnt)
else:
    print(-1)