#!/bin/bash
":" //#; docker run --rm -it -v "$(cd $(dirname $0) && pwd)":/vo:ro  node:5.2-slim /bin/bash -c "node /vo/$(basename $0) < /vo/g.inp"; exit $?

"use strict";
let main = (input) => {
    let x = Number.parseInt(input);
    let ans = ":(";
    for(let i=0; i<x; i++){
	if(Math.floor(i*1.08)===x) {
	    ans = i;
	    break;
	}
    }
    return ans;
};
console.log(main(require("fs").readFileSync("/dev/stdin", "utf-8").trim()));
