import java.util.Scanner;

/**
 * B - TAKOYAKI FESTIVAL 2019
 */
public class Main {

    public static void main(String[] args) {

        Scanner sc = new Scanner( System.in );
        int n = sc.nextInt();   // たこ焼きの数
        int[] d = new int[n];   // 美味しさ


        for( int i = 0; i < n; i++ ) {
            d[i] = sc.nextInt();
        }

        int sum = 0;
        for( int j = 0; j < n; j++ ) {
            for( int k = j; k < ( n - 1 ); k++ ) {
                sum += d[j] * d[k + 1];
            }
        }

        System.out.println( sum );

        sc.close();
    }
}
/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