import java.util.*;

public class Main {

    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int N = scanner.nextInt();
        int[] d = new int[N];

        for(int i=0; i<N; i++){
            d[i] = scanner.nextInt();
        }

        long ans = 0;
        for(int i=0; i<N-1; i++){
            for(int j=i+1; j<N; j++){
                ans += d[i] * d[j];
            }
        }

        System.out.println(ans);

    }

}
/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