const main = input =>{
    const nums = input.split('').map(n=>Number(n));
    const o_l = nums.filter(n=>n===1).length;
    const z_l = nums.filter(n=>n===0).length;
    let ans = 0;
    if (z_l > o_l){
        ans=o_l;
    } else if (z_l <= o_l) {
        ans=z_l;
    }
    console.log(ans*2);
};
main(require('fs').readFileSync('/dev/stdin', 'utf8'));