function Main(N) {
var O =  Math.floor(N/1.08);
  if(Math.floor(O*1.08) == N){
    console.log(O);
  }else if(Math.floor((O+1)*1.08) == N){
    console.log(O+1);
  }else{
    console.log(":(")
  }
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));