import java.util.Scanner;
public class Main {
	public static void main(String[] args) {
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		Long arr[]=new Long[n];
		for(int i=0;i<n;i++) {
			arr[i]=sc.nextLong();
		}
		System.out.println(c(arr));
	}

	public static Long c(Long arr[]) {
		Long sum=0L;
		if(arr.length==0) {
			return sum;
		}		
		for(int i=0;i<arr.length;i++) {
			for(int j=0;j<i;j++) {
				sum+=arr[i]*arr[j];
			}			 
		}
		return sum;
	}	
}