#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll=int;
using ld=long double;
ld pie=3.141592653589793;
ll inf=69;
ll mod=998244353;
int main(){
    ll n;
    cin >> n;
    string s;
    cin >> s;
    for (ll i = 1; i < s.size(); i++)
    {
        if (s[i]!='?'&&s[i]==s[i-1])
        {
            cout << 0 << endl;
            return 0;
        }
        if (i>=2)
        {
            if (s[i]!='?'&&s[i]==s[i-2])
            {
                cout << 0 << endl;
                return 0;
            }
        }
    }
    ll ans=0;
    vector<vector<vector<ll>>>dp(n,vector<vector<ll>>(26,vector<ll>(26,0)));
    if (s[0]=='?')
    {
        for (ll i = 0; i < 26; i++)
        {
            dp[0][i][i]=1;
        }
    }else{
        dp[0][s[0]-'a'][s[0]-'a']=1;
    }
    for (ll i = 1; i < s.size(); i++)
    {
        if (s[i]!='?')
        {
            for (ll j = 0; j < 26; j++)
            {
                if (j==s[i]-'a')
                {
                    continue;
                }
                for (ll k = 0; k < 26; k++)
                {
                    if (k==s[i]-'a')
                    {
                        continue;
                    }
                    dp[i][s[i]-'a'][j]+=dp[i-1][j][k];
                    if (dp[i][s[i]-'a'][j]>mod)
                    {
                        dp[i][s[i]-'a'][j]-=mod;
                    }
                }
            }
        }else{
            for (ll l = 0; l < 26; l++)
            {
                for (ll j = 0; j < 26; j++)
                {
                    if (j==l)
                    {
                        continue;
                    }
                    for (ll k = 0; k < 26; k++)
                    {
                        if (k==l)
                        {
                            continue;
                        }
                        dp[i][l][j]+=dp[i-1][j][k];
                        if (dp[i][l][j]>mod)
                        {
                            dp[i][l][j]-=mod;
                        }
                    }
                }
            }
        }
    }
    for (ll i = 0; i < 26; i++)
    {
        for (ll j = 0; j < 26; j++)
        {
            ans+=dp[n-1][i][j];
            ans%=mod;
        }
    }
    cout << ans << endl;
}