import java.util.Scanner;
public class Main{
    public static void main(String []args){
        Scanner scan = new Scanner(System.in);
        int n = scan.nextInt();
        int []array = new int[n+1];
        int []prefixSum = new int[n+1]; //前缀和数组
        int sum = 0;
        for(int i = 1;i < n + 1;i++){
            array[i] = scan.nextInt();
        }
        prefixSum[1]=array[1];
        for(int i = 2;i < n+1;i++){
            prefixSum[i] = prefixSum[i-1]+array[i];
        }
        for(int i = 1;i < n + 1;i++){
            sum += array[i]*(prefixSum[n]-prefixSum[i]);
        }
        System.out.println(sum);
    }

}