#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int ans = 0;
  int c0, c1;
  for (int i = 0; i < s.size(); i++) {
    if (s[i] == '0') c0++;
    if (s[i] == '1') c1++;
  }
  cout << min(c0, c1) * 2 << endl;
}
