import sys,random,bisect
from collections import deque,defaultdict
from heapq import heapify,heappop,heappush
from itertools import permutations
from math import gcd,log

from math import sqrt, ceil
from bisect import bisect_left, bisect_right
from typing import Iterable


input = lambda :sys.stdin.readline().rstrip()
mi = lambda :map(int,input().split())
li = lambda :list(mi())

class SegmentTree:
    def __init__(self, init_val, segfunc, ide_ele):
        n = len(init_val)
        self.segfunc = segfunc
        self.ide_ele = ide_ele
        self.num = 1 << (n - 1).bit_length()
        self.tree = [ide_ele] * 2 * self.num
        self.size = n
        for i in range(n):
            self.tree[self.num + i] = init_val[i]
        for i in range(self.num - 1, 0, -1):
            self.tree[i] = self.segfunc(self.tree[2 * i], self.tree[2 * i + 1])

    def update(self, k, x):
        k += self.num
        self.tree[k] = x
        while k > 1:
            k >>= 1
            self.tree[k] = self.segfunc(self.tree[2*k], self.tree[2*k+1])

    def query(self, l, r):
        if r==self.size:
            r = self.num

        res = self.ide_ele

        l += self.num
        r += self.num
        right = []
        while l < r:
            if l & 1:
                res = self.segfunc(res, self.tree[l])
                l += 1
            if r & 1:
                right.append(self.tree[r-1])
            l >>= 1
            r >>= 1

        for e in right[::-1]:
            res = self.segfunc(res,e)
        return res

def merge(x,y):
    return (x[0]+y[0],x[1]+y[1],x[2]+y[2]+x[0]*y[1])

N = int(input())
S = [s for s in input()]

idx = [i for i in range(N) if S[i]=="?"]
init = [-1] * N
for i in range(N):
    if S[i]=="A":
        init[i] = (1,0,0)
    else:
        init[i] = (0,1,0)
seg = SegmentTree(init,merge,(0,0,0))

res = seg.tree[1][2]
for i in idx:
    seg.update(i,(1,0,0))
    res = max(res,seg.tree[1][2])
print(res)