两份代码，加了 unsigned 才能过？

82分代码

#include<bits/stdc++.h>
using namespace std;
long int n,sum;
int main()
{
    cin>>n;
    sum=n*(n-1)/2*(n-2)/3*(n-3)/4;
    cout<<sum;
    return 0;
}

AC代码

#include<bits/stdc++.h>
using namespace std;
unsigned long int n,sum;
int main()
{
    cin>>n;
    sum=n*(n-1)/2*(n-2)/3*(n-3)/4;
    cout<<sum;
    return 0;
}