int s(int n, int k){
    vector <vector <int>> S(n + 1, vector <int>(k + 1, 0));
    S[0][0] = 1;
    for (int i = 1; i < n + 1; i++){
        for (int j = 1; j < k + 1; j++){
            S[i][j] = j * S[i - 1][j] + S[i - 1][j - 1];
        }
    }
    return S[n][k];
}