n = int(input())
s = input()
now = 0
a_left = 0
for c in s:
    if c == 'A':
        a_left += 1
    else:
        now += a_left
ans = now
c_right = n - a_left
a_left = 0
for i in range(n):
    if s[i] == 'A':
        a_left += 1
    elif s[i] == 'C':
        c_right -= 1
    else:
        now -= a_left
        c_right -= 1
        a_left += 1
        now += c_right
    ans = max(ans, now)
print(ans)