#include <bits/stdc++.h>
using namespace std;
int main() {
  string S;
  cin >> S;
  int count = 0;
  int k = 0;
  while (S[k]) {
    count++;
    k++;
  }
  int henka = 0;
  int kai = 0;
  int tmp = count;
  for (int j = 0; j < tmp; j++) {
    henka = 0;
    for (int i = 0; i < count - 1; i++) {
      if (S[i] != S[i + 1] && S[i] != 2 && S[i + 1] != 2) {
        for (int l = 0; i + 2 + l < count; l++) {
          S[i + l] = S[i + 2 + l];
        }
        count -= 2;
        kai += 2;
        i++;
        henka = 1;
      }
    }
    if (henka == 0) {
      break;
    }
  }
  cout << kai;
}
