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 = 0;

 for(i=1; i<=n; i++){
   var tmp = Math.floor(i * 1.08);
   if(tmp  == n){
     ans++;
     console.log(i);
   }
 }

 if(ans == 0){
   console.log(":(");
 }
});