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

idx = { "A": 0, "C": 1, "?": 2 }

dp = [[0 for _ in range(3)] for _ in range(n)]
for i in range(n - 2, -1, -1):
  for c in range(3):
    dp[i][c] = dp[i + 1][c]
  dp[i][idx[s[i + 1]]] += 1

cur = 0
for i in range(n):
  if s[i] == "A":
    cur += dp[i][1] + dp[i][2]

ans = cur
cnt = 0
for i in range(n):
  if s[i] == "?":
    cur -= cnt
    cur += dp[i][1] + dp[i][2]
    ans = max(ans, cur)
    cnt += 1
  elif s[i] == "A":
    cnt += 1

print(ans)