参考答案：

#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
const int N = 50010;
int st[N];
int sum[N];
int primes[N];
int cnt;
vector<int> res;
int a, b;

void get_primes(int n)
{
    for (int i = 2; i <= n; ++i)
    {
        if (!st[i]) primes[++cnt] = i;
        for (int j = 1; primes[j] <= n / i; ++j)
        {
            st[primes[j] * i] = 1;
            if (i % primes[j] == 0) break;
        }
    }
}
int get(int a, int b)
{
    int res = 0;
    while (a)
    {
        res += a / b;
        a /= b;
    }
    return res;
}
vector<int> mul(vector<int> a, int b)
{
    reverse(a.begin(), a.end());
    vector<int> c;
    int t = 0;
    for (int i = 0; i < a.size(); ++i)
    {
        t += a[i] * b;
        c.push_back(t % 10);
        t /= 10;
    }
    while (t)
    {
        c.push_back(t % 10);
        t /= 10;
    }
    reverse(c.begin(), c.end());
    return c;
}
int main()
{
    cin >> a >> b;
    get_primes(a);

    for (int i = 1; i <= cnt; ++i)
    {
        int p = primes[i];
        sum[i] = get(a, p) - get(b, p) - get(a - b, p);
    }
    res.push_back(1);
    for (int i = 1; i <= cnt; ++i)
    {
        for (int j = 0; j < sum[i]; ++j)
        {
            res = mul(res, primes[i]);
        }
    }
    for (int i = 0; i < res.size(); ++i) printf("%d", res[i]);
    cout << endl;
    return 0;
}