P2181 diagonal

For one nn A convex polygon with vertices , None of its three diagonals meet at one point . Request the number of diagonal intersections in the drawing .
Input format
Enter only one line and one integer nn, Represents the number of edges .

Output format
Output a line with an integer representing the answer .

I/o sample
Input #1 Copy
3
Output #1 Copy
0
Input #2 Copy
6
Output #2 Copy
15

import java.math.BigInteger;
import java.util.*;
/*  Mathematical derivation , When the vertex is less than 4 There can be no intersection , When the vertex equals four, there is and only one , that   When the vertex is larger than four, we can combine the vertices , Application Cn To see how many possibilities there are ！ */
public class Main{
    
    public static void main(String [] args){
    
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        // The range of basic data types cannot accept all data , So here we need to use two classes of infinity BigInteger( integer ) and BigDecimal（ floating-point ）
        BigInteger bd1 = new BigInteger(String.valueOf(n));
        BigInteger bd2 = new BigInteger(String.valueOf(n-1));
        BigInteger bd3 = new BigInteger(String.valueOf(n-2));
        BigInteger bd4 = new BigInteger(String.valueOf(n-3));
        BigInteger bd5 = new BigInteger("24");

        bd1 = bd1.multiply(bd2);
        bd1 = bd1.multiply(bd3);
        bd1 = bd1.multiply(bd4);
        if(n<=3)
            System.out.println(0);
        else {
    
            bd1 = bd1.divide(bd5);
            System.out.println(bd1);
        }
    }
}