## https://yukicoder.me/problems/no/2276




def main():
    N = int(input())
    S = input()

    # 全てAにした時の回答
    answer = 0
    a_num = 0
    for s in S:
        if s in ("A", "?"):
            a_num += 1
        else:
            answer += a_num

    max_answer = answer
    c_num = 0
    for s in reversed(S):
        if s == "?":
            a_num -= 1
            answer -= c_num
            answer += a_num
            c_num += 1
        elif s == "C":
            c_num += 1
        elif s == "A":
            a_num -= 1
        max_answer = max(max_answer, answer)
    print(max_answer)



if __name__ == "__main__":
    main()