#include <iostream>
#include <iomanip>
#include <cstdio>
#include <queue>
#include <cstring>
#include <vector>
#include <map>
#include <algorithm>
#include <cctype>
#include <cmath>
#include <bitset>
#include <set>
#include <deque>

using namespace std;

#define REP(i,n) for(int i=0;i<(int)(n);i++)
#define RREP(i,n) for(int i=n-1;i>=0;i--)
#define FOR(i,k,n) for(int i=(k);i<(int)(n);i++)
#define all(i,n) (i),(i+n)

int dx4[4]={1,0,-1,0};
int dy4[4]={0,-1,0,1};
int dx8[8]={1,0,-1,1,-1,1,0,-1};
int dy8[8]={1,1,1,0,0,-1,-1,-1};

typedef pair<int, int> P;
typedef pair<string, int> SP;
typedef long long ll;
typedef pair<ll, ll> PLL;

const int INF = 1e9;
const ll LLINF = 1e18;
const int MAX_V = 1e6+1;
const ll mod = 1000000007;

// --------------------------------------
string s;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);

  cin >> s;
  int a[400005];
  REP(i, 400005) a[i] = -1;
  a[200000] = 0;

  int sum = 200000, ans = 0;
  REP(i, s.length()) {
    if(s[i] == 'A') sum++;
    else sum--;

    if(a[sum] == -1) a[sum] = i + 1;
    else ans = max((i + 1) - a[sum], ans);
  }

  cout << ans << endl;
}