N=int(input())
S=list(input())
AC=[False]*N
for i in range(N):
    if S[i]=="?":
        AC[i]=True
        S[i]="C"
cnt=0
a=0
for s in S:
    if s=="A":
        a+=1
    else:
        cnt+=a
ans=cnt
a=0
c=S.count("C")
for i in range(N):
    if AC[i]:
        S[i]="A"
        c-=1
        cnt-=a
        cnt+=c
    if S[i]=="A":
        a+=1
    else:
        c-=1
    ans=max(ans,cnt)
print(ans)