# 前半はAがいい、後半はCがいい
# なぜならCが出るたびに、それまでのAのカウントがansに追加されるから
# dpなのか貪欲的なのか
# dp[i個目][Aの個数][今回A0 C1] 得点最大値

N = int(input())
S = input()
INF = 10**10
dp = [[[-INF]*2 for j in range(N+1)] for i in range(N+1)]
dp[0][0][0] = 0
dp[0][0][1] = 0
#print(dp[0])

for i in range(1, N+1):
    if S[i-1] == 'A':
        for j in range(N+1):
            if j+1 <= N:
                dp[i][j+1][0] = max(dp[i][j+1][0], dp[i-1][j][0])
                dp[i][j+1][0] = max(dp[i][j+1][0], dp[i-1][j][1])
    elif S[i-1] == 'C':
        for j in range(N+1):
            if j <= N:
                dp[i][j][1] = max(dp[i][j][1], dp[i-1][j][0]+j)
                dp[i][j][1] = max(dp[i][j][1], dp[i-1][j][1]+j)        
    elif S[i-1] == '?':
        for j in range(N+1):
            if j+1 <= N:
                dp[i][j+1][0] = max(dp[i][j+1][0], dp[i-1][j][0])
                dp[i][j+1][0] = max(dp[i][j+1][0], dp[i-1][j][1])
        for j in range(N+1):
            if j <= N:
                dp[i][j][1] = max(dp[i][j][1], dp[i-1][j][0]+j)
                dp[i][j][1] = max(dp[i][j][1], dp[i-1][j][1]+j)  
    #print(dp[i])

ans = 0
for j in range(N+1):
    for k in range(2):
        ans = max(ans, dp[N][j][k])
print(ans)