n = int(input())
s = input()

tmp = 0
A = 0
C = 0
for c in s:
    if c == 'A':
        A += 1
    else:
        C += 1
        tmp += A
ans = tmp
A = 0
for c in s:
    if c == 'A':
        A += 1
    elif c == '?':
        C -= 1
        tmp -= A
        tmp += C
        ans = max(ans, tmp)
        A += 1
    else:
        C -= 1
print(ans)