using System;
using System.Collections.Generic;
using System.Collections;
using System.Collections.Specialized;
using System.Linq;
using System.Text;
using System.IO;
using System.Reflection;
using static System.Math;
using System.Numerics;
using System.Threading;
using System.Runtime.CompilerServices;
using System.Diagnostics;
//using nint=System.Int32;
static class Program{
	const int mod=998244353;
	static void Main(){
		Sc sc=new Sc();
		var n=sc.I;
		var t=sc.S;
		var dp=new long[2][][];
		const int m=26;
		dp[1]=new long[m+1][];
		for(int j = 0;j<m;j++) {dp[1][j]=new long[m+2];}
		dp[1][m]=Enumerable.Repeat(1L,m+2).ToArray();
		for(int i = 0;i<n;i++) {
			dp[i&1]=new long[m+1][];
			for(int j = 0;j<=m;j++) {dp[i&1][j]=new long[m+2];}
			if(t[i]=='?'){
				for(int j = 0;j<m;j++) {
					for(int k = 0;k<=m;k++) {
						if(j==k){continue;}
						dp[i&1][j][k]=mod+dp[(i&1)^1][k][0]-dp[(i&1)^1][k][j]+dp[(i&1)^1][k][j+1];
						dp[i&1][j][k]%=mod;
					}
					for(int k = m;k>=0;k--) {
						dp[i&1][j][k]+=dp[i&1][j][k+1];
						dp[i&1][j][k]%=mod;
					}
				}
			}
			else{
				int j=t[i]-'a';
				for(int k = 0;k<=m;k++) {
					if(j==k){continue;}
					dp[i&1][j][k]=mod+dp[(i&1)^1][k][0]-dp[(i&1)^1][k][j]+dp[(i&1)^1][k][j+1];
					dp[i&1][j][k]%=mod;
				}
				for(int k = m;k>=0;k--) {
					dp[i&1][j][k]+=dp[i&1][j][k+1];
					dp[i&1][j][k]%=mod;
				}
			}
		}
		long ans=0;
		if(t[n-1]=='?'){
			for(int i = 0;i<m;i++) {ans+=dp[(n&1)^1][i][0];}
			ans%=mod;
		}
		else{ans=dp[(n&1)^1][t[n-1]-'a'][0];}
		Console.WriteLine("{0}",ans);
	}
}

public class Sc{
	public int I{get{return int.Parse(Console.ReadLine());}}
	public long L{get{return long.Parse(Console.ReadLine());}}
	public double D{get{return double.Parse(Console.ReadLine());}}
	public string S{get{return Console.ReadLine();}}
	public int[] Ia{get{return Array.ConvertAll(Console.ReadLine().Split(),int.Parse);}}
	public long[] La{get{return Array.ConvertAll(Console.ReadLine().Split(),long.Parse);}}
	public double[] Da{get{return Array.ConvertAll(Console.ReadLine().Split(),double.Parse);}}
	public string[] Sa{get{return Console.ReadLine().Split();}}
	public object[] Oa{get{return Console.ReadLine().Split();}}
	public int[] Ia2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),int.Parse);}}
	public int[] Ia3(string a,string b){return Array.ConvertAll((a+Console.ReadLine()+b).Split(),int.Parse);}
	public int[] Ia3(int a){return Array.ConvertAll((Console.ReadLine()+" "+a.ToString()).Split(),int.Parse);}
	public long[] La2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),long.Parse);}}
	public long[] La3(string a,string b){return Array.ConvertAll((a+Console.ReadLine()+b).Split(),long.Parse);}
	public long[] La3(int a){return Array.ConvertAll((Console.ReadLine()+" "+a.ToString()).Split(),long.Parse);}
	public double[] Da2{get{return Array.ConvertAll(("0 "+Console.ReadLine()+" 0").Split(),double.Parse);}}
	public double[] Da3(string a,string b){return Array.ConvertAll((a+Console.ReadLine()+b).Split(),double.Parse);}
	public T[] Arr<T>(int n,Func<T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f();}return a;}
	public T[] Arr<T>(int n,Func<int,T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i);}return a;}
	public T[] Arr<T>(int n,Func<string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(Console.ReadLine().Split());}return a;}
	public T[] Arr<T>(int n,Func<int,string[],T> f){var a=new T[n];for(int i=0;i<n;i++){a[i]=f(i,Console.ReadLine().Split());}return a;}
	public (T,T) Tp2<T>(){var s=Console.ReadLine().Split();return (Ct<T>(s[0]),Ct<T>(s[1]));}
	public (T,T,T) Tp3<T>(){var s=Console.ReadLine().Split();return (Ct<T>(s[0]),Ct<T>(s[1]),Ct<T>(s[2]));}
	public (T,T,T,T) Tp4<T>(){var s=Console.ReadLine().Split();return (Ct<T>(s[0]),Ct<T>(s[1]),Ct<T>(s[2]),Ct<T>(s[3]));}
	public (T1,T2) Tp2<T1,T2>(){var s=Console.ReadLine().Split();return (Ct<T1>(s[0]),Ct<T2>(s[1]));}
	public (T1,T1,T2) Tp3<T1,T2>(){var s=Console.ReadLine().Split();return (Ct<T1>(s[0]),Ct<T1>(s[1]),Ct<T2>(s[2]));}
	private T Ct<T>(string s){return (T)Convert.ChangeType(s,typeof(T));}
}