import java.io.*;
import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner cin = new Scanner(System.in);
        PrintWriter cout = new PrintWriter(System.out);

        int n = cin.nextInt();
        int[] d = new int[n];
        for(int i=0;i<n;i++) d[i] = cin.nextInt();
        long sum = 0;
        for(int i=0;i<n;i++) {
            for(int j=i+1;j<n;j++) {
                sum += d[i] * d[j];
            }
        }

        cout.println(sum);
        cout.close();
    }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz