#include<iostream>
#include<atcoder/modint>
using namespace std;
using mint=atcoder::modint998244353;
int N;
string S;
mint dp[2<<17][2][2];
main()
{
	cin>>N>>S;
	dp[0][0][0]=1;
	for(int i=0;i<N;i++)
	{
		char c=S[i];
		for(int j=0;j<2;j++)for(int k=0;k<2;k++)
		{
			char lim=j?'z':c;
			for(char t=k?'b':'a';t<=lim;t++)
			{
				dp[i+1][j|t<lim][k|t=='a']+=dp[i][j][k];
			}
		}
	}
	cout<<dp[N][1][1].val()<<endl;
}