import java.util.*;

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