n,k = map(int,raw_input().split())
a = map(int,raw_input().split())
b = [sorted(a).index(i) for i in a]
cnt = 0
for i in range(n):
    if b[i]%k != i%k:
        cnt = -1
if cnt == 0:
    for i in range(n):
        j = b.index(i)
        c = cmp(i,j)
        cnt += abs(i-j)/k
        while i != j:
	        b[j], b[j+k*c] = b[j+k*c], b[j]
	        j = j+k*c
print cnt