import java.util.*;
public class Main {
public static void main(String[] args){
Scanner s=new Scanner(System.in);
  int n=s.nextInt();
 int tako[]=new int[n];
  for(int i=0;i<n;i++){
  tako[i]=s.nextInt();
  }
  int cnt=0;
 r: for(int i=n-1;i>-1;i--){
  for(int k=i-1;k>-1;k--){
    if(i<=0)
      break r;
  cnt+=tako[i]*tako[k];
  }
  }
  System.out.println(cnt);
}
}
/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