"use strict";

const main = arg => {
    let N = arg.split("\n")[0].split("");  
    const red = N.filter(n=>n=="0").length;
    const blue = N.length - red;
 
    console.log(Math.min(red, blue) * 2);
    
}

main(require('fs').readFileSync('/dev/stdin', 'utf8'));