N = int(input())
S = [0]+list(input())+[0]
A = [0]*(N+2)
X = []
for i in range(1,N+1):
    if S[i]=="A":
        A[i] = A[i-1]+1
    else:
        A[i] = A[i-1]
    if S[i]=="?":
        X.append(i)
C = [0]*(N+2)
for i in range(N,0,-1):
    if S[i]=="C":
        C[i] = C[i+1]+1
    else:
        C[i] = C[i+1]
ans = 0
for i in range(1,N+1):
    if S[i]=="C" or S[i]=="?":
        ans += A[i]
M = len(X)
cnt = ans
for j in range(M):
    i = X[j]
    cnt -= A[i]+j
    cnt += C[i]+(M-j-1)
    ans = max(ans,cnt)
print(ans)