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