#include #include #include #include #include #include #include #include #include #include #include #include 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>> dp(N, vector(26, vector(26))); vector 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