#include <bits/stdc++.h>
using namespace std;

int main() {
    string s;
    cin >> s;
    int n = s.size();
    unordered_map<int, int> first;
    first[0] = -1;
    int prefix = 0, ans = 0;
    for (int i = 0; i < n; i++) {
        prefix += (s[i] == 'A') ? 1 : -1;
        if (first.count(prefix)) {
            ans = max(ans, i - first[prefix]);
        } else {
            first[prefix] = i;
        }
    }
    cout << ans << endl;
    return 0;
}

import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        int n = s.length();
        Map<Integer, Integer> first = new HashMap<>();
        first.put(0, -1);
        int prefix = 0, ans = 0;
        for (int i = 0; i < n; i++) {
            prefix += (s.charAt(i) == 'A') ? 1 : -1;
            if (first.containsKey(prefix)) {
                ans = Math.max(ans, i - first.get(prefix));
            } else {
                first.put(prefix, i);
            }
        }
        System.out.println(ans);
    }
}

def main():
    s = input().strip()
    first = {0: -1}
    prefix = 0
    ans = 0
    for i, c in enumerate(s):
        prefix += 1 if c == 'A' else -1
        if prefix in first:
            ans = max(ans, i - first[prefix])
        else:
            first[prefix] = i
    print(ans)

main()

const readline = require('readline');
const rl = readline.createInterface({ input: process.stdin });

rl.on('line', (line) => {
    const s = line.trim();
    const first = new Map();
    first.set(0, -1);
    let prefix = 0, ans = 0;
    for (let i = 0; i < s.length; i++) {
        prefix += s[i] === 'A' ? 1 : -1;
        if (first.has(prefix)) {
            ans = Math.max(ans, i - first.get(prefix));
        } else {
            first.set(prefix, i);
        }
    }
    console.log(ans);
    rl.close();
});