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