#include <bits/stdc++.h>
const int INF = 1e9;
const long long LINF = 1e18;
const int MOD = 1e9 + 7;
using namespace std;
int main() {
  string s;
  cin >> s;
  int red = 0, blue = 0;
  for (int i = 0; i < s.size() + 1; i++) {
    if (s[i] == '0')
      red++;
    else
      blue++;
  }
  cout << min(red, blue) * 2 << endl;
}
