def calc_tax_ex(tax_in, rate)
  tax_ex = tax_in / rate
  return tax_ex.floor.to_i if (tax_ex.floor * rate).floor == tax_in
  return tax_ex.floor.to_i + 1 if ((tax_ex.floor + 1) * rate).floor == tax_in
  ':('
end

puts calc_tax_ex(gets.to_i, 1.08)