#include <bits/stdc++.h>
using namespace std;
int main() {
  int count0, count1 = 0;
  string st;
  cin >> st;
  for (int i = 0; i < (int)st.size(); i++) {
    if (st.at(i) == '1') {
      count1++;
    } else {
      count0++;
    }
  }
  if (count0 >= count1) {
    cout << count1 * 2 << endl;
  } else {
    cout << count0 * 2 << endl;
  }
}
