#include <bits/stdc++.h>
using namespace std;
int main() {
  int cnt = 0, flag = 1;
  string s;
  cin >> s;
  while (flag) {
    flag = 0;
    for (int i = 0; i < s.size() - 1; i++) {
      if (s[i] != s[i + 1]) {
        s.erase(s.begin() + i);
        s.erase(s.begin() + i);
        cnt = cnt + 2;
        flag = 1;
        i--;
      }
    }
  }
  cout << cnt << endl;
  return 0;
}
