import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int N = sc.nextInt();
        int a[] = new int[100];
        int sum = 0;
        for(int i = 0; i < N; i++) {
        	a[i] = sc.nextInt();
        }
        for(int i = 0; i < N; i++) {
        	for(int j = i+1; j < N; j++) {
        		sum += a[i] * a[j];
        	}
        }
        System.out.println(sum);
        }
    
}
/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