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();
        if (n == 1)
        {
            if (s[0] == '?') WriteLine(26);
            else WriteLine(1);
            return;
        }
        if (n == 2)
        {
            if (s[0] == '?' && s[1] == '?') WriteLine(26 * 25);
            else if (s[0] == '?' || s[1] == '?') WriteLine(25);
            else if (s[0] == s[1]) WriteLine(0);
            else WriteLine(1);
            return;
        }
        var dp = new long[26 * 26];
        for (var i = 0; i < 26; ++i) for (var j = 0; j < 26; ++j)
        {
            if (i == j) continue;
            if ((s[0] == '?' || s[0] - 'a' == i) && (s[1] == '?' || s[1] - 'a' == j)) dp[i * 26 + j] = 1;
        }
        var mod = 998_244_353;
        for (var i = 2; i < n; ++i)
        {
            var sum = new long[26];
            for (var j = 0; j < dp.Length; ++j) sum[j % 26] = (sum[j % 26] + dp[j]) % mod;
            var ndp = new long[dp.Length];
            for (var j = 0; j < dp.Length; ++j)
            {
                var x = j / 26;
                var y = j % 26;
                if (x == y) continue;
                if (s[i] == '?' || s[i] - 'a' == y) ndp[j] = (sum[x] - dp[y * 26 + x] + mod) % mod;
            }
            dp = ndp;
        }
        WriteLine(dp.Sum() % mod);
    }
}