import sys
input = sys.stdin.readline
mod = 998244353

N = int(input())
S = input().rstrip('\n')

T = []
for s in S:
    if s == "?":
        T.append("C")
    else:
        T.append(s)
T = "".join(T)
ans = 0
c = T.count("C")
for t in T:
    if t == "A":
        ans += c
    else:
        c -= 1
tmp = ans
c = T.count("C")
a = 0
for i, t in enumerate(T):
    s = S[i]
    if t == "C":
        c -= 1
    else:
        a += 1
    if s == "?":
        tmp += c
        tmp -= a
        ans = max(ans, tmp)
        a += 1
print(ans)