#include <stdio.h>
#include <string.h>
#define S_MAX 200001

int main() {
    char s[S_MAX];
    scanf("%s", s);
    int len = strlen(s);

    int a[S_MAX] = { 0 }, b[S_MAX] = { 0 };
    for (int i = len - 1; i >= 0; --i) {
        if (s[i] == 'A') ++a[i];
        else if (s[i] == 'B') ++b[i];
        a[i] += a[i + 1];
        b[i] += b[i + 1];
    }

    int ans = 0;
    for (int i = 0; i < len - 1; ++i) {
        int cnt = b[i] - a[i];
        for (int j = len - 1; j >= i; --j) {
            if (cnt == 0) {
                if (j - i + 1 > ans) ans = j - i + 1;
                break;
            }
            if (s[j] == 'A') ++cnt;
            else if (s[j] == 'B') --cnt;
        }
    }
    
    printf("%d\n", ans);
    return 0;
}