import std.algorithm;
import std.array;
import std.conv;
import std.math;
import std.range;
import std.stdio;
import std.string;

void nosol () {
  writeln (":(");
}

void main() {
  int n;
  readf (" %d", &n);
  //n = floor (x * 1.08)
  foreach (x; 1 .. n + 1) {
    if ( (x * 108) / 100 == n) {
      writeln (x);
      return;
    }
  }
  nosol ();
}

