N=int(input())
S=input()
A,C,B=[],[],[];a,c,b=0,0,0
D=[]
for i in range(N):
  if S[i]=='A':
    a+=1
  elif S[i]=='C':
    c+=1 
  else:
    b+=1
    D.append(i)
  A.append(a);C.append(c);B.append(b)

ans=0
for i in range(N):
  if S[i]!='A':
    ans+=A[i]

now=ans;ss=S.count('?')
bl,br=0,ss-1
for i in range(ss):
  d=D[i]
  al,cr=A[d],C[-1]-C[d]
  now+=br+cr-al-bl
  ans=max(ans,now)
  bl+=1;br-=1
print(ans)