#include <bits/stdc++.h>
using namespace std;
using ll = long long;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  string s;
  cin >> s;
  int a, b;
  for (int x = 0; x < (int)(s.size()); x++) {
    if (s[x] == '0') a++;
    if (s[x] == '1') b++;
  }
  cout << min(a, b) * 2 << endl;
}
