#include <bits/stdc++.h>
using namespace std;
int main(void) {
  string s;
  cin >> s;
  int c0 = 0, c1 = 0;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == '0')
      c0++;
    else
      c1++;
  }
  if (c1 == 0 || c0 == 0) {
    cout << 0 << endl;
  } else if (c1 == c0) {
    cout << c1 * 2 << endl;
  } else
    cout << abs(c1 - c0) * 2 << endl;
}
