using System;
using static System.Console;
using System.Linq;
using System.Collections.Generic;
class Program
{
    static int NN => int.Parse(ReadLine());
    static int[] NList => ReadLine().Split().Select(int.Parse).ToArray();
    public static void Main()
    {
        Solve();
    }
    static void Solve()
    {
        var n = NN;
        var s = ReadLine();

        var sc = s.Replace('?', 'C');
        var acount = 0;
        var tmp = 0L;
        for (var i = 0; i < n; ++i)
        {
            if (sc[i] == 'A') ++acount;
            else tmp += acount;
        }
        var ccount = new int[n];
        for (var i = n - 1; i >= 0; --i)
        {
            if (i < n - 1) ccount[i] = ccount[i + 1];
            if (sc[i] != 'A') ++ccount[i];
        }
        acount = 0;
        var max = tmp;
        for (var i = 0; i < n; ++i)
        {
            if (s[i] == 'A') ++acount;
            if (s[i] == '?')
            {
                tmp = tmp - acount + ccount[i] - 1;
                ++acount;
                max = Math.Max(max, tmp);
            }
        }
        WriteLine(max);
    }
}