#include <bits/stdc++.h>
using namespace std;
int ctoi(const char c) {
  switch (c) {
    case '0':
      return 0;
    case '1':
      return 1;
    case '2':
      return 2;
    case '3':
      return 3;
    case '4':
      return 4;
    case '5':
      return 5;
    case '6':
      return 6;
    case '7':
      return 7;
    case '8':
      return 8;
    case '9':
      return 9;
    default:
      return -1;
  }
}
int main() {
  string a;
  int sum = 0;
  int flag = 0;
  vector<int> v;
  cin >> a;
  for (int i = 0; i < a.size(); i++) {
    v.push_back(ctoi(a[i]));
  }
  int length;
  length = v.size();
  while (1) {
    int tmp;
    tmp = v.at(0);
    for (int i = 0; i < v.size(); i++) {
      if (tmp != v.at(i)) {
        if (v.size() != 2) {
          v.erase(v.begin() + i - 1);
          v.erase(v.begin() + i);
          sum += 2;
          flag = 1;
          break;
        } else {
          sum += 2;
          flag = 0;
          break;
        }
      } else {
        flag = 0;
        tmp = v.at(i);
      }
    }
    if (flag == 0) {
      break;
    }
  }
  cout << sum << endl;
  return 0;
}
