#include <bits/stdc++.h>
using namespace std;
int main() {
  int red = 0, blue = 0;
  string c;
  cin >> c;
  for (int i = 0; i <= c.size(); i++) {
    if (c[i] == '0')
      red++;
    else
      blue++;
  }
  cout << 2 * min(red, blue) << endl;
  return 0;
}
