#robot
N,K=(int(i) for i in input().split())
a=[int(i) for i in input().split()]
b=a.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+=K
        C+=1
    if j>=N:
        C=-1
    i+=1
print(C)