const input = require('fs').readFileSync('/dev/stdin', 'utf8');
 
const arr = input.split('');

let num1 = 0;
 
let num0 = 0;
 
for (let i=0;i<arr.length;i++) {
  if (arr[i] === '1') {
    num1++;
  } else {
    num0++;
  }
}
 
if (num1 < num0) {
  console.log(num1 * 2);
} else {
  console.log(num0 * 2);
}