import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {

    static long largestValue(int[] A) {
        // Return the largest value of any of A's nonempty subarrays.
    
        int sum=0;
        for(int i=1;i<5;i++)
        {
            for(int j=i;j<5;j++)
            {
                if(i==j)break;
                
                sum=sum+(A[i]*A[j]);
            }
        }
        long res=sum;
        return res;
        
        
    }

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int[] A = new int[n];
        for(int A_i = 0; A_i < n; A_i++){
            A[A_i] = in.nextInt();
        }
        long result = largestValue(A);
        System.out.println(result);
        in.close();
    }
}