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