N,K=map(int,input().split())
A=list(map(int,input().split()))
B=[]
for i in range(N):
    B.append((A[i],i))
list.sort(B)
ok=True
for i in range(N):
    if B[i][1]%K!=i%K:
        ok=False
if not ok:
    print(-1)
    exit()
def bubble(x):
    res=0
    for i in range(len(x)):
        for j in range(len(x)-1):
            if x[j]>x[j+1]:
                x[j],x[j+1]=x[j+1],x[j]
                res+=1
    return res
ans=0
for i in range(K):
    ans+=bubble(A[i::K])
print(ans)