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

CR = [0] * (N+1)
for i in reversed(range(N)):
    tmp = 0
    if S[i] == "?" or S[i] == "C":
        tmp = 1
    CR[i] = CR[i+1] + tmp

ans = 0
for i in range(N):
    if S[i] == "A":
        ans += CR[i+1]
cnta = 0
tmp = ans
for i in range(N):
    if S[i] == "A":
        cnta += 1
    elif S[i] == "?":
        tmp -= cnta
        tmp += CR[i+1]
        cnta += 1
        ans = max(ans,tmp)
print(ans)