N = gets.to_i
X = gets.split.map(&:to_i)
M = gets.to_i

m = X[M - 1]
ave = Rational(X.sum, N)
ave2 = Rational(X.map { |x| (x - ave) ** 2 }.sum, N)

v = Math.sqrt(ave2)

if v == 0
  puts 50
else
  diff = m - ave
  d = (10 * diff) / v
  b = (50 + d)

  if b < 0
    puts (50 + d).ceil
  else
    puts (50 + d).floor
  end
end