int record_choose[MAXN][MAXM];
int choose(int n, int m) {
    if (m == 0 || m == n) {
        return 1;
    }
    if (record_choose[n][m] == 0)
        record_choose[n][m] = choose(n - 1, m - 1) + choose(n - 1, m);
    return record_choose[n][m];
}