n, k = map(int,input().split())
a = list(map(int, input().split()))
ans = 0
for i in range(n):
    for j in range(n-i-k):
        if a[j] > a[j+k]:
            a[j], a[j+k] = a[j+k], a[j]
            ans += 1
print(ans if a == sorted(a) else -1)