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

    A_cnt=S.count("A")
    C_cnt=S.count("C")
    Q_cnt=S.count("?")

    score=0
    rc=C_cnt+Q_cnt
    k=rc
    for i in range(N):
        if S[i]=="A":
            score+=k
        else:
            k-=1

    ans=score
    la=0
    for i in range(N):
        if S[i]=="A":
            la+=1
        elif S[i]=="C":
            rc-=1
        else:
            rc-=1
            score+=rc-la
            la+=1

            ans=max(ans, score)
    return ans

#==================================================
print(solve())