import sys
from collections import deque, Counter
input = lambda: sys.stdin.readline().rstrip()
ii = lambda: int(input())
mi = lambda: map(int, input().split())
li = lambda: list(mi())
inf = 2 ** 63 - 1
mod = 998244353

n = ii()
s = input()
counta = 0
countc = 0
ans = 0
for v in s:
    if v == 'C' or v == '?':
        ans += counta
        countc += 1
    else:
        counta += 1

counta = 0
now = ans
for v in s:
    if v == 'A':
        counta += 1
    if v == 'C':
        countc -= 1
    if v == '?':
        countc -= 1
        now += countc
        now -= counta
        ans = max(ans, now)
        counta += 1
print(ans)