import sys
input = sys.stdin.readline

N = int(input())
S = input().rstrip()


right = [0] * (N+1)
cnt = 0
for i in range(N)[::-1]:
    right[i] = right[i+1]
    if S[i] == 'A':
        right[i] += cnt
    else:
        cnt += 1


left = [0] * (N+1)
cnt = 0
for i in range(N):
    left[i+1] = left[i]
    if S[i] == 'C':
        left[i+1] += cnt
    else:
        cnt += 1

ans = 0
A = 0
C = 0
for c in S:
    if c != 'A':
        C += 1

for i in range(N+1):
    ans = max(ans, left[i] + right[i] + A * C)
    if i < N:
        if S[i] == 'A':
            A += 1
        elif S[i] == '?':
            A += 1
            C -= 1
        else:
            C -= 1

print(ans)