"use strict"
const create = () => {
    const input = require("fs").readFileSync("/dev/stdin", "utf8").trim().split("\n");
    const res = {
        "list": input, "index": 0, "max": input.length,
        "hasNext": function () { return this.index < this.max },
        "next": function () { if (!this.hasNext()) { throw "ArrayIndexOutOfBoundsException"; } else { return this.list[this.index++]; } }
    };
    return res;
}
const o = create();
const next = () => o.next()
const nextInt = () => parseInt(o.next())
const nextStrArray = () => o.next().split(" ")
const nextIntArray = () => o.next().split(" ").map(el => parseInt(el))
const nextCharArray = () => o.next().split("")
const hasNext = () => o.hasNext()
const myout = (...x) => console.log(...x)
const Main = () => {
    let arr = next().split("")
    let ans = 0
    while (true) {
        let tmp = arr.length
        for (let i = 0; i < arr.length-1; i++) {
            if (arr[i] == '0' && arr[i+1] == '1') {
                arr.splice(i, 2)
                ans += 2
                break
            } else if (arr[i] == '1' && arr[i+1] == '0') {
                arr.splice(i, 2)
                ans += 2
                break
            }
        }
        if(tmp == arr.length){
            console.log(ans)
            return
        }
    }
}
Main()

