N,K=map(int, input().split())
A=list(map(int, input().split()))
SA = sorted(A)
cnt = 0
for i in range(N-K-1):
    for j in range(0,N-K-1):
        if A[j] > A[j+K]:
            A[j],A[j+K]=A[j+K],A[j]
            cnt+=1
for i in range(N):
    if SA[i]!=A[i]:
        print(-1)
        exit()
print(cnt)