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];
    for (int i=0;i<N;i++){
      d[i] = sc.nextInt();
    }
    int ans = 0;
    for(int i=0;i<N;i++){
      for(int j=i+1;j<N;j++){
        if (i==j){ continue; }
        ans = 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