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