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