#include<iostream>
using namespace std;
int main()
{
   unsigned long long n;
    cin >> n;
    if (n == 4)
        cout << 1;
    else
        cout << n * (n - 1) / 2 * (n - 2) / 3 * (n - 3) / 4;
}

为什么这样写可以通过？

#include<iostream>
using namespace std;
int main()
{
   unsigned long long n;
    cin >> n;
    if (n == 4)
        cout << 1;
    else
        cout << n * (n - 1)  * (n - 2) * (n - 3) / 24;
}
```但这样写不行了