local read = io.read
local min = math.min

local n, k = read("n", "n")
local a_t = {}
local min_num = math.maxinteger
for i = 1, n do
	local a_i = read("n")
	a_t[i] = a_i
	min_num = min(min_num, a_i)
end

local not_min_amount = 0
for i = 1, n do
	if a_t[i] ~= min_num then
		not_min_amount = not_min_amount + 1
	end
end

print(math.ceil(not_min_amount / (k - 1)))
