N,K = map(int,input().split())
A = list(map(int,input().split()))
A1 = A[:]
A2 = sorted(A)
invNum = 0
for k in range(K):
    B = []
    for i in range(k,N,K):
        B.append(A[i])
    n = len(B)
    for i in range(n-1,-1,-1):
        b = B[i]
        cnt = 0
        for j in range(i-1,-1,-1):
            if B[j]>b:
                cnt += 1
        invNum += cnt
    B = sorted(B)
    j = 0
    for i in range(k,N,K):
        A1[i] = B[j]
        j += 1
if A2==A1:
    print(invNum)
else:
    print(-1)