N = int(input())
S = list(input())
ans = 0
cnt = 0
for i in range(N):
    if S[i] == "A" or S[i] == "?":
        cnt += 1
    else:
        ans += cnt
        
num = ans
cntC = 0
for i in range(N - 1, -1, -1):
    if S[i] == "?":
        cnt -= 1
        num += cnt
        num -= cntC
        cntC += 1
    elif S[i] == "C":
        cntC += 1
    else:
        cnt -= 1
    ans = max(ans, num)
print(ans)