/* -*- coding: utf-8 -*-
 *
 * 672.cc: No.672 最長AB列 - yukicoder
 */

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<set>
#include<stack>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<numeric>
#include<utility>
#include<complex>
#include<functional>
 
using namespace std;

/* constant */

const int MAX_N = 200000;
const int MAX_M = MAX_N * 2 + 1;

/* typedef */

/* global variables */

int minis[MAX_M], maxis[MAX_M];

/* subroutines */

/* main */

int main() {
  string s;
  cin >> s;
  int n = s.size();

  int m = n * 2 + 1;
  int c = n;

  memset(minis, -1, sizeof(minis));
  minis[c] = 0;

  for (int i = 0; i < n; i++) {
    if (s[i] == 'A') c++;
    else c--;

    if (minis[c] < 0) minis[c] = i + 1;
    maxis[c] = i + 1;
  }

  int maxl = 0;
  for (int i = 0; i < m; i++)
    if (minis[i] >= 0) {
      int l = maxis[i] - minis[i];
      if (maxl < l) maxl = l;
    }

  printf("%d\n", maxl);
  return 0;
}