import java.io.*;
import java.util.*;

class Main
{
    public static void main(String args[])throws Exception
    {
        BufferedReader bu=new BufferedReader(new InputStreamReader(System.in));
        StringBuilder sb=new StringBuilder();
        int n=Integer.parseInt(bu.readLine());
        char s[]=bu.readLine().toCharArray();
        int i,cur=1;
        long dp[][][]=new long[2][26][26],sum[]=new long[26],M=998244353;
        if(n==1)
        {
            int ans=1;
            if(s[0]=='?') ans=26;
            System.out.println(ans);
            return;
        }

        if(n>1)
        {
            int j,v,v2;
            if(s[1]=='?') v=(1<<26)-1;
            else v=1<<(s[1]-'a');
            if(s[0]=='?') v2=(1<<26)-1;
            else v2=1<<(s[0]-'a');

            for(i=0;i<26;i++)
            if(((v>>i)&1)==1)
            for(j=0;j<26;j++)
            if(j!=i && ((v2>>j)&1)==1) dp[0][i][j]++;
        }

        for(i=2;i<n;i++)
        {
            int j,k,v;
            if(s[i]=='?') v=(1<<26)-1;
            else v=1<<(s[i]-'a');
            for(j=0;j<26;j++)
            {
                sum[j]=0;
                Arrays.fill(dp[cur][j],0);
            }

            for(j=0;j<26;j++)
            for(k=0;k<26;k++)
            sum[j]=(sum[j]+dp[cur^1][j][k])%M;

            for(j=0;j<26;j++)
            if(((v>>j)&1)==1)
            {
                for(k=0;k<26;k++)
                if(k!=j) dp[cur][j][k]=(dp[cur][j][k]+(sum[k]-dp[cur^1][k][j]+M)%M)%M;
            }
            cur^=1;
        }

        long ans=0;
        for(i=0;i<26;i++)
        for(int j=0;j<26;j++)
        ans=(ans+dp[cur^1][i][j])%M;
        System.out.println(ans);
    }
}