#robot
N,K=(int(i) for i in input().split())
a=[int(i) for i in input().split()]
b=[0]*N
for i in range(N):
    b[i]=a[i]
b.sort()
i=0
C=0
while i<N and C>=0:
    j=i
    while j<N and C>=0 and b[j]!=a[i]:
        j=i
        while j<N and C>=0 and b[j]!=a[i]:
            j+=K
    #        print(C)
        if j>=N:
            C=-1
        else:
            l=0
            while i+l+K<=j:
                t=a[i+l]
                a[i+l]=a[i+l+K]
                a[i+l+K]=t
                l+=K
                C+=1
   #     print(a)
    i+=1
 #   print(a)
print(C)