# 3次元dpはMLEした
# 1次元目と3次元目を交換してみる

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

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

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