#coding: utf-8

inputN, inputK = map(int, input().split())
inputA = list(map(int, input().split()))
minNumber = inputN
minPos = 0

if inputN == inputK:
    print (1)
elif inputN <= (inputK-1)*100000 + 1:
    for i in range(2, 100001):
        if inputN <= (inputK-1)*i + 1:
            print (i)
            break
else:

    for i in range(len(inputA)):
        if inputA[i] < minNumber:
            minNumber = inputA[i]
            minPos = i

    splitNumber = (inputN + 1) / (inputK - 1)
    print (int(splitNumber))
