N = int(input())
S = input()

ans = 0

lsc = []
cnt = 0
for i in range(N):
    if S[i] == 'A':
        cnt += 1
    elif S[i] == '?':
        lsc.append(cnt)
    else:
        ans += cnt
lsc.reverse()
acc = [0]
for i in range(len(lsc)):
    acc.append(acc[-1] + lsc[i])

lsa = []
cnt = 0
for i in range(N - 1, -1, -1):
    if S[i] == 'C':
        cnt += 1
    elif S[i] == '?':
        lsa.append(cnt)
lsa.reverse()
aca = [0]
for i in range(len(lsa)):
    aca.append(aca[-1] + lsa[i])

aca.reverse()


L = len(lsa)
fans = ans
for i in range(L + 1):
    fans = max(fans, ans + acc[i] + aca[i] + i * (L - i))

print(fans)