n = int(input())
S = list(input())

ans = 0
a_cnt = 0
for i in range(n):
    if S[i] == 'A' or S[i] == '?':
        a_cnt += 1
    else:
        ans += a_cnt
c_cnt = 0
res = ans
for i in range(n - 1, -1, -1):
    if S[i] == 'A':
        a_cnt -= 1
    elif S[i] == 'C':
        c_cnt += 1
    elif S[i] == '?':
        a_cnt -= 1
        res -= c_cnt
        res += a_cnt
        ans = max(ans, res)
        c_cnt += 1
print(ans)