#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int n = s.size();
  int z = 0, o;
  for (int i = 0; i < (n); i++) {
    if (s[i] == '0') z++;
  }
  o = n - z;
  cout << abs(o - z) << endl;
}
