#include <iostream>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <iomanip>
#include <queue>
#include <algorithm>
#include <numeric>
#include <deque>
#include <complex>
#include <cassert>

using namespace std;
using ll = long long;

const ll modc = 998244353;
class mint {
    ll x;
public:
    mint(ll x=0) : x((x%modc+modc)%modc) {}
    mint operator-() const {
      return mint(-x);
    }
    mint& operator+=(const mint& a) {
        if ((x += a.x) >= modc) x -= modc;
        return *this;
    }
    mint& operator-=(const mint& a) {
        if ((x += modc-a.x) >= modc) x -= modc;
        return *this;
    }
    mint& operator*=(const  mint& a) {
        (x *= a.x) %= modc;
        return *this;
    }
    mint operator+(const mint& a) const {
        mint res(*this);
        return res+=a;
    }
    mint operator-(const mint& a) const {
        mint res(*this);
        return res-=a;
    }
    mint operator*(const mint& a) const {
        mint res(*this);
        return res*=a;
    }
    mint pow(ll t) const {
        if (!t) return 1;
        mint a = pow(t>>1);
        a *= a;
        if (t&1) a *= *this;
        return a;
    }
    mint inv() const {
        return pow(modc-2);
    }
    mint& operator/=(const mint& a) {
        return (*this) *= a.inv();
    }
    mint operator/(const mint& a) const {
        mint res(*this);
        return res/=a;
    }
    bool operator == (const mint& a) const{
        return x == a.x;
    }
    friend ostream& operator<<(ostream& os, const mint& m){
        os << m.x;
        return os;
    }
    friend istream& operator>>(istream& ip, mint &m) {
        ll t;
        ip >> t;
        m = mint(t);
        return ip;
    }
};

int main(){

    int N;
    string S;
    cin >> N >> S;
    if (N == 1){
        cout << (S[0] == '?' ? 26 : 1) << endl;
        return 0;
    }
    vector<vector<vector<mint>>> dp(N, vector(26, vector<mint>(26)));
    vector<mint> sm(26);
    mint ans=0;

    for (int i=0; i<26; i++){
        for (int j=0; j<26; j++){
            if (i == j) continue;
            if (S[0] != '?' && S[0]-'a' != j) continue;
            if (S[1] != '?' && S[1]-'a' != i) continue;
            dp[1][i][j] = 1;
            sm[i] += 1;
        }
    }

    for (int i=2; i<N; i++){
        for (int j=0; j<26; j++){
            if (S[i] != '?'){
                if (j != S[i]-'a') dp[i][S[i]-'a'][j] += sm[j] - dp[i-1][j][S[i]-'a'];
            }
            else{
                for (int l=0; l<26; l++){
                    if (j != l) dp[i][l][j] += sm[j] - dp[i-1][j][l];
                }
            }
        }
        for (int j=0; j<26; j++){
            sm[j] = 0;
            for (int k=0; k<26; k++) sm[j] += dp[i][j][k];
        }
    }
    for (int i=0; i<26; i++){
        for (int j=0; j<26; j++) ans += dp[N-1][i][j];
    }
    cout << ans << endl;
}