import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################

n = ni()
s = input()
A = [0]
C = [0]
B = [0]
ans = 0
r = 0
for i in range(n):
    A.append(A[i])
    B.append(B[i])
    C.append(C[i])
    if s[i] == "A":
        A[-1] += 1
    elif s[i] == "C":
        C[-1] += 1
    else:
        B[-1] += 1
    if s[i] != "A":
        r += A[i]
ans = r
#print(r)
for i in range(n):
    if s[i] == "?":
        r -= A[i] + B[i]
        r += C[-1] - C[i] + B[-1] - B[i] - 1
        #print(r)
        ans = max(ans, r)

print(ans)