def calc(X):
    M = len(X)
    val = 0
    for i in range(M):
        for j in range(i + 1, M):
            if X[i] > X[j]:
                val += 1
    return val

N, K = map(int, input().split())
A = list(map(int, input().split()))
L = [[] for i in range(K)]
for i in range(N):
    L[i%K].append(A[i])

ans = 0
for i in range(K):
    ans += calc(L[i])
    
D = []
for i in range(K):
    L[i].sort(reverse=True)

for i in range(N):
    D.append(L[i%K].pop())
A.sort()
if A == D:
    print(ans)
else:
    print(-1)