N,K=map(int,raw_input().split())
a=map(int,raw_input().split())
A=[(a[i],i) for i in range(N)]
A.sort()
for i,(p,q) in enumerate(A):
    if N<=K and i!=q:
        print -1
        exit()
    a[q]=i

ans=0
for i in range(K):
    for j in range(i,N,K):
        for k in range(j+K, N, K):
            if a[j]%K != i or a[k]%K !=i:
                print -1
                exit()
            elif a[j]>a[k]:
                ans += 1
print ans