#include <bits/stdc++.h>
using namespace std;
int main() {
  stack<int> s1;
  stack<int> s2;
  string s;
  long long sum1, sum2;
  cin >> s;
  for (long long i = 0; i < s.length(); i++) {
    s1.push(s[i] - '0');
  }
  sum1 = s1.size();
  if (sum1 != 1) {
    while (s1.empty() != true) {
      int cpy = s1.top();
      s1.pop();
      int cpy1 = s1.top();
      s1.pop();
      if (cpy != cpy1) {
        s2.push(cpy);
        s2.push(cpy1);
      } else {
        while (s2.empty() != true) {
          s1.push(s2.top());
          s2.pop();
        }
      }
    }
    sum2 = s2.size();
    cout << sum1 - sum2;
  } else
    cout << "0";
  return 0;
}
