function main(arg) {
  const price = parseInt(arg)
  const a = Math.ceil(price / 1.08)
  const b = Math.floor(price / 1.08)

  if (Math.floor(a * 1.08) === price) {
    console.log(a)
  } else if (Math.floor(b * 1.08) === price) {
    console.log(b)
  } else {
    console.log(':(')
  }
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'))