#include <bits/stdc++.h>
using namespace std;

struct iofast_t {
    iofast_t() {
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
    }
} iofast;

struct uns_t {} uns;
template <typename Element, typename Head, typename ...Args>
auto vec(Element init, Head arg, Args ...args) {
    if constexpr (sizeof...(Args) == 0) return vector(arg, init);
    else return vector(arg, vec(init, args...));
}
template <typename Element, typename Head, typename ...Args>
auto vec(uns_t, Head arg, Args ...args) {
    return vec(Element(), arg, args...);
}

template <typename Container>
auto distance(const Container &c, decltype(begin(c)) iter) {
    return distance(begin(c), iter);
}

template <typename RIter, typename Compare = less<typename iterator_traits<RIter>::value_type>>
auto isort(RIter first, RIter last, Compare comp = Compare()) {
    vector<int> i(distance(first, last));
    iota(begin(i), end(i), 0);
    sort(begin(i), end(i), [&](auto x, auto y) {
        return comp(*(first + x), *(first + y));
    });
    return i;
}

template <typename, template <typename> typename, typename = void_t<>>
struct detect : false_type {};
template <typename T, template <typename> typename Check>
struct detect<T, Check, void_t<Check<T>>> : true_type {};
template <typename T, template <typename> typename Check>
constexpr inline bool detect_v = detect<T, Check>::value;

template <typename T>
using has_member_sort = decltype(declval<T>().sort());

template <typename Container, typename Compare = less<typename Container::value_type>>
auto sorted(Container c, Compare comp = Compare()) {
    if constexpr (detect_v<Container, has_member_sort>) {
        c.sort(comp);
        return c;
    }
    else {
        sort(begin(c), end(c), comp);
        return c;
    }
}

template <typename Container, typename Compare = equal_to<typename Container::value_type>>
auto uniqued(Container c, Compare comp = Compare()) {
    c.erase(unique(begin(c), end(c), comp), end(c));
    return c;
}

template <typename T, typename Compare = less<T>>
T &chmin(T &l, T r, Compare &&f = less<T>()) { return l = min(l, r, f); }
template <typename T, typename Compare = less<T>>
T &chmax(T &l, T r, Compare &&f = less<T>()) { return l = max(l, r, f); }

template <typename F>
constexpr auto fix(F &&f) noexcept {
    return [f = std::tuple<F>(std::forward<F>(f))](auto &&...args) mutable {
        return std::get<0>(f)(fix(std::get<0>(f)), std::forward<decltype(args)>(args)...);
    };
}

#include <atcoder/modint>
using mint = atcoder::modint998244353;

constexpr int lim = 50000;
constexpr int alpha = 'z' - 'a' + 1;

mint dp[lim][alpha][alpha];

int main() {
    int n; cin >> n;
    string s; cin >> s;

    if (n == 1) {
        cout << (s[0] == '?' ? alpha : 1) << endl;
        return 0;
    }

    for (char c = 'a'; c <= 'z'; ++c) {
        if (!(s[0] == c || s[0] == '?')) {
            continue;
        }
        for (char d = 'a'; d <= 'z'; ++d) {
            if (!(s[1] == d || s[1] == '?') || c == d) {
                continue;
            }
            dp[1][c - 'a'][d - 'a'] = 1;
        }
    }

    for (int i = 2; i < n; ++i) {
        for (char c = 'a'; c <= 'z'; ++c) {
            for (char d = 'a'; d <= 'z'; ++d) {
                if (c == d) {
                    continue;
                }
                for (char e = 'a'; e <= 'z'; ++e) {
                    if (e == c || e == d) {
                        continue;
                    }
                    if (!(s[i] == e || s[i] == '?')) {
                        continue;
                    }
                    dp[i][d - 'a'][e - 'a'] += dp[i - 1][c - 'a'][d - 'a'];
                }
            }
        }
    }

    mint ans = 0;

    for (char c = 'a'; c <= 'z'; ++c) {
        for (char d = 'a'; d <= 'z'; ++d) {
            ans += dp[n - 1][c - 'a'][d - 'a'];
        }
    }

    cout << ans.val() << endl;
}