# ?の数を数えて、前半何個までをAとするかで全探索してみるか

N = int(input())
S = input()
q_count = S.count('?')

ans = 0
for q in range(q_count+1):
    subans = 0
    A_count = 0
    used = 0
    for i in range(N):
        if S[i] == 'A':
            A_count += 1
        elif S[i] == 'C':
            subans += A_count
        elif S[i] == '?':
            if used < q:
                # treat as A
                used += 1
                A_count += 1
            else: 
                # treat as C
                subans += A_count
    #print('q', q, 'subans', subans)
    ans = max(ans, subans)
print(ans)