#include <iostream>
#include <fstream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <cmath>
#include <unordered_map>
#include <map>
#include <queue>
#include <string>
#include <set>
#include <list>
#include <climits>
#include <bitset>
#include <numeric>
#include <cassert>
#include <regex>
using std::cout;
using std::cin;
using std::string;
using std::vector;

struct pos
{
    int first, last;
    pos()
    {
        first = last = -1;
    }
};

int main()
{
    string s;
    cin >> s;
    std::map<int, pos> arr;
    arr[0].first = 0;
    arr[0].last = 0;
    int sum = 0;
    for (int i = 0; i < s.size(); i++)
    {
        if (s[i] == 'A')
        {
            sum++;
        }
        else
        {
            sum--;
        }
        if (arr[sum].first == -1)
        {
            arr[sum].first = i+1;
        }
        arr[sum].last = i+1;
    }
    int max = 0;
    for (const auto [k, p] : arr)
    {
        if (p.first == -1)
        {
            continue;
        }
        max = std::max(max, p.last - p.first);
    }
    cout << max;
}