from copy import deepcopy
N, K = map(int, input().split())
A = list(map(int, input().split()))
B = sorted(list(set(A)))
D = dict(zip(B,range(len(B))))

for i in range(N):
    A[i] = D[A[i]]

B = [[] for _ in range(K)]
for i in range(N):
    B[i % K].append(A[i])
C = deepcopy(B)

for b in B:
    b.sort()
now = 0
for i in range(N):
    b = B[i%K][i//K]
    if now > b:
        print(-1)
        exit()
    now = b

from atcoder.fenwicktree import FenwickTree
ans = 0
for cs in C:
    if cs:
        NC = max(cs)
        ft = FenwickTree(NC + 1)
        cnt = 0
        for c in cs:
            cnt += ft.sum(c, NC + 1)
            ft.add(c, 1)
        ans += cnt
print(ans)