process.stdin.resume();
process.stdin.setEncoding('utf8');
// 自分の得意な言語で
// Let's チャレンジ！！

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

var a = parseInt(lines[0],10);

var aa = a/1.08;

if(Number.isInteger(aa)){
    console.log(aa);
}else{
    var bb = Math.ceil(aa);
    var cc = Math.floor(aa);
    if(Math.floor(bb*1.08)===a){
        console.log(bb);
    }else if(Math.floor(cc*1.08)===a){
       console.log(cc);}
       else{
           console.log(':(')
       }
}


});