"use strict";
const main = input => {
    const s = input[0];
    let red = 0, blue = 0;
    for(let i=0; i<s.length; i++) {
        if (s[i] == 0) {
            red++;
        } else {
            blue++;
        }
    }
    console.log(Math.min(red, blue)*2);
    
}   
main(require("fs").readFileSync("/dev/stdin", "utf8").split('\n'));