#include <iostream>
#include <string>
#include <cstring>
#include <cstdlib>
#include <cmath>
#include <algorithm>
#include <vector>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <list>
#include <iterator>
#include <cassert>
#include <numeric>
#include <functional>
#include <ctime>
#include <bitset>
#pragma warning(disable:4996) 

#define ATCODER
#ifdef ATCODER
#include <atcoder/all>
#endif

typedef long long ll;
typedef unsigned long long ull;
#define LINF  9223300000000000000
#define LINF2 1223300000000000000
#define LINF3 1000000000000
#define INF 2140000000
//const long long MOD = 1000000007;
const long long MOD = 998244353;

using namespace std;
#ifdef ATCODER
using namespace atcoder;
#endif

char str[50005];

void solve()
{
    int n;
    scanf("%d%s", &n, str);
    vector<ll> v(26 * 26);
    v[0] = 1;
    for (int i = 0; i < n; i++) {
        vector<ll> v2(26 * 26);
        for (int j = 0; j < 26 * 26; j++) {
            if (v[j] == 0) continue;

            int i0 = j / 26, i1 = j % 26;
            for (int k = 0; k < 26; k++) {
                if (str[i] != '?' && str[i] - 'a' != k) continue;
                if ((i>1 && i0 == k) || (i>0 && i1 == k)) continue;

                int j2=i1 * 26 + k;
                v2[j2] = (v2[j2]+v[j])%MOD;
            }
        }
        v.swap(v2);
    }
    ll ans = 0;
    for (int j = 0; j < 26 * 26; j++) {
        ans = (ans + v[j])%MOD;
    }
    printf("%lld\n", ans);
    return;
}

int main()
{
#if 1
    solve();
#else
    int T, t;
    scanf("%d", &T);
    for (t = 0; t < T; t++) {
        //printf("Case #%d: ", t + 1);
        solve();
    }
#endif
    return 0;
}