#include <bits/stdc++.h>
using namespace std;
int main() {
  string s;
  cin >> s;
  int count = 0;
TRAVEL:
  for (int i = 0; i < s.size() - 1; i++) {
    if (s[i] != s[i + 1]) {
      s.erase(i, 2);
      count += 2;
      goto TRAVEL;
    }
  }
  cout << count;
}
