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