process.stdin.resume();
process.stdin.setEncoding('utf8');

var lines = []
var reader = require('readline').createInterface({
  input: process.stdin,
  output: process.stdout
});

reader.on('line', (line) => {
  lines.push(line);
});

reader.on('close', () => {
 var n = Number(lines[0]);
 var ans = n / 1.08;
 if(Number.isInteger(ans) == true){
   console.log(ans);
 }else if(Number.isInteger(ans) == false){
   if(Math.floor(ans*10) % 10 == 0){
     console.log(":(");
   }else{
     console.log(Math.round(ans));
   }
 }
});