import sys

def count(arr):
    inv = 0
    n = len(arr)
    for i in range(n):
        for j in range(i + 1, n):
            if arr[i] > arr[j]:
                inv += 1
    return inv

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

b = sorted(a)

ans = 0

for start in range(k):
    cur = []
    need = []
    
    i = start
    while i < n:
        cur.append(a[i])
        need.append(b[i])
        i += k
    
    if sorted(cur) != sorted(need):
        print(-1)
        sys.exit()
    
    ans += count(cur)

print(ans)