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

tmp = 0
la = 0
for c in s:
  if c == 'A':
    la += 1
  else:
    tmp += la

ans = tmp
la = 0
rc = s[1:n].count('C') + s[1:n].count('?')
for i in range(n):
  if s[i] == '?':
    tmp -= la
    tmp += rc
    ans = max(ans, tmp)
  if s[i] != 'C':
    la += 1
  if i < n - 1 and s[i + 1] != 'A':
    rc -= 1
print(ans)