import java.util.*;

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

    int all = 0;    
    for(int i=0; i<N; i++){
        d[i] = sc.nextInt();
}
    for(int i=0; i<N-1; i++){
        for(int j=i+1; j<N; j++){
    all += d[i]*d[j];        
    }
}
	System.out.println(all);
      }
}

/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