"use strict";
    
const main = arg => {
    arg = arg.trim().split("\n");
    const N = parseInt(arg[0]);
    
    const temp = N / 1.08;
    
    // console.log(temp)
    
    const floor = Math.floor(temp * 1.08);
    const ceil  = Math.ceil(temp) * 1.08;
    
    // console.log(999 * 1.08)
    if(floor === N) {
        // console.log(Math.floor(temp))
        if(Math.floor(temp) * 1.08 === N) {
            console.log(Math.floor(temp))
        } else if(Math.floor(Math.ceil(temp) * 1.08) === N) {
            console.log(Math.ceil(temp))
        } else {
            console.log(":(")
        }
    } else {
        console.log(":(")
    }
}
main(require('fs').readFileSync('/dev/stdin', 'utf8'));