bill_division.rb

#!/bin/ruby

require 'json'
require 'stringio'

# Complete the bonAppetit function below.
def bonAppetit(bill, k, b)
  sum = bill.inject(:+)
  together_sum = sum - bill[k]
  each_share = together_sum / 2

  if each_share == b
    puts "Bon Appetit"
    else puts b - each_share
  end
end

nk = gets.rstrip.split

n = nk[0].to_i

k = nk[1].to_i

bill = gets.rstrip.split.map(&:to_i)

b = gets.strip.to_i

bonAppetit bill, k, b