只用了6ms

#include<bits/stdc++.h>
using namespace std;

int main()
{
    int n,sum=0;
    cin >> n;
    for (int a=1;a<=n/3;a++)
    {
        for (int b=a+1;b<n;b++)
        {
            int c=n-a-b;
            if (c>b)
                sum++;
        }
    }
    cout << sum;
    return 0;
}