def f(A):
    N=len(A)
    x=0
    for i in range(N):
        for j in range(N-1):
            if A[j]>A[j+1]:
                A[j],A[j+1]=A[j+1],A[j]
                x+=1
    return x

N,K=map(int,input().split())
A=list(map(int,input().split()))

B=[[] for _ in range(K)]
t=0
for a in A:
    B[t].append(a)
    t=(t+1)%K

A.sort()

X=0
for b in B:
    X+=f(b)

B=[sorted(x)[::-1] for x in B]
C=[]
t=0
for i in range(N):
    x=B[t].pop()
    C.append(x)
    t=(t+1)%K

print(X if A==C else -1)