"use strict";
const Main = function(input){
   let xs = input.trim().split(' ').map(v=>parseInt(v,10));
   let tax =1.08;
   let ans1 =[];
   let ans2 = 0;
   for(let i =0 ; i<=xs[0]; i++){
      ans1.push(Math.floor(i*tax));
      if(ans1[i]==xs[0]){
         ans2 = Math.ceil(xs[0]/tax);
         break;
      }else{
         ans2 = -1;
      }
   } 
   if(ans2 != -1){
     console.log(ans2);
    }else{
     console.log(":(");
    }
}
Main(require('fs').readFileSync('/dev/stdin','utf8'));