import numpy as np
import math
n, k = map(int,input().split())
a = list(map(int,input().split()))
start = [i for i,x in enumerate(a) if x == min(a)]
m = 10000000
for i in start:
    if math.ceil(i/(k-1))+math.ceil(((n-1)-i)/(k-1)) < m:
        m = math.ceil(i/(k-1))+math.ceil(((n-1)-i)/(k-1))
print(m)