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