#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <iomanip>
#include <cmath>
#include <stdio.h>
#include <queue>
#include <deque>
#include <cstdio>
#include <set>
#include <map>
#include <bitset>
#include <stack>
#include <cctype>
using namespace std;
int co[400040];
int a[200020], b[200020];
int main() {
	co[0] = 0;
	string s;
	cin >> s;
	int n = s.size();
	for (int i = 0; i <= 400040; i++) {
		co[i] = -1;
	}
	for (int i = 1; i <= n; i++) {
		if (s[i - 1] == 'A') {
			a[i]++;
		}
		else {
			b[i]++;
		}
		a[i] += a[i - 1];
		b[i] += b[i - 1];
	}
	int ans = 0;
	for (int i = 0; i <= n; i++) {
		if (co[a[i] - b[i] + 200000] == -1) {
			co[a[i] - b[i] + 200000] = i;
		}
		else {
			if (ans < i - co[a[i] - b[i] + 200000]) {
				ans = i - co[a[i] - b[i] + 200000];
			}
		}
	}
	cout << ans << endl;
	return 0;
}