import java.io.*;
class Main{
  public static void main(String args[]) throws IOException{
    BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
    int n = Integer.parseInt(br.readLine());
    int a[]=new int[n];
    String input = br.readLine();
    String b[]=input.split(" ");
    for(int i=0;i<a.length;i++){
      a[i]=Integer.parseInt(b[i]);
    }
    int sum=0;
    for(int i=0;i<n;i++){
      for(int j=i+1;j<n;j++){
        sum = 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