###スニペット始まり###
import sys, re
from copy import copy, deepcopy
from math import ceil, floor, sqrt,factorial, gcd, pi, degrees, radians, sin, asin, cos, acos, tan, atan2
from collections import Counter, deque, defaultdict
from heapq import heapify, heappop, heappush
from itertools import permutations, accumulate, product, combinations, combinations_with_replacement
from bisect import bisect, bisect_left, bisect_right
from functools import reduce, lru_cache
from string import ascii_uppercase, ascii_lowercase
from decimal import Decimal, ROUND_HALF_UP #四捨五入用

def input(): return sys.stdin.readline().rstrip('\n')
#easy-testのpypyでは再帰数を下げる。
if __file__=='prog.py':
    sys.setrecursionlimit(10**5)
else:
    sys.setrecursionlimit(10**6)

def lcm(a, b): return a * b // gcd(a, b)

#3つ以上の最大公約数/最小公倍数。Nを要素数、Mを数値の大きさとして、O(NlogM)
def gcd_v2(l: list): return reduce(gcd, l)
def lcm_v2(l: list): return reduce(lcm, l)

#nPk
def nPk(n, k): return factorial(n) // factorial(n - k)

#逆元
def modinv(a, mod=10**9+7): return pow(a, mod-2, mod)
INF = float('inf')
MOD = 10 ** 9 + 7
###スニペット終わり###

N=int(input())
S=list(input())

hatena_idx=[]
for i, s in enumerate(S):
    if s=='?':
        hatena_idx.append(i)

max_=-INF
def is_ok(mid):
    global max_
    s=S
    for i, idx in enumerate(hatena_idx):
        if i<mid:
            s[idx]='A'
        else:
            s[idx]='C'
    counter=Counter()

    cnt=0
    for i in range(N):
        if s[-(i+1)]=='A':
            cnt+=counter['C']
        counter[s[-(i+1)]]+=1
    
    max_=max(cnt,max_)
    return max_==cnt

def meguru_bisect(ng, ok):

    while (abs(ok - ng) > 1):
        mid = (ok + ng) // 2
        if is_ok(mid):
            ok = mid
        else:
            ng = mid
    return ok

meguru_bisect(len(hatena_idx)+1, -1)
print(max_)