import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        String n[]=reader.readLine().split(" ");
        String str[]=reader.readLine().split(" ");
        //关闭扫描器
        reader.close();
        //初始化
        long  all=0;
        int a[]=new int[str.length+1];
        int s[]=new int[str.length+1];
        for (int i = 1; i < str.length+1 ; i++) {
            a[i]=Integer.parseInt(str[i-1]);
            s[i]=a[i]+s[i-1];
        }
        for (int i = 1; i < s.length-1; i++) {
            all+=a[i+1]*s[i];
        }
        System.out.println(all);
    }

}