"use strict";
const Main = function(input){
   let xs = input.trim().split(' ').map(v=>parseInt(v,10));
   let tax =1.08;
   let ans1=Math.floor(xs[0]/tax);  
   if(ans1*tax==xs[0]||(ans1+1)*tax==xs[0]){
   console.log(ans1);
   }else{
   console.log(":(");
   }  
}
Main(require('fs').readFileSync('/dev/stdin','utf8'));