import java.io.*;
import java.math.*;
import java.util.*;
@SuppressWarnings("Duplicates")
// author @mdazmat9
public class Main{
    static Scanner sc = new Scanner(System.in);
    static PrintWriter out = new PrintWriter(System.out);
    public static void main(String[] args) throws IOException {
        int test = 1;
      //   test=sc.nextInt();
        for (int ind = 0; ind < test; ind++) {
            solve();
        }
        out.flush();
    }

    static void solve() {
        int n=sc.nextInt();
        int[]a =intarray(n);
        long ans=0;
        for(int i=0;i<n;i++){
            for(int j=i+1;j<n;j++){
                long temp=a[i]*a[j];
                ans+=temp;
            }
        }
        out.println(ans);

    }


    static int[] intarray(int n){ int [] a=new int[n];for(int i=0;i<n;i++)a[i]=sc.nextInt();return a; }
    static void sort(int[]a){ shuffle(a);Arrays.sort(a);}
    static void sort(long[]a){ shuffle(a);Arrays.sort(a);}
    static long[] longarray(int n){ long [] a=new long[n];for(int i=0;i<n;i++)a[i]=sc.nextLong();return a; }
    static ArrayList<Integer> intlist(int n){ArrayList<Integer> list=new ArrayList<>();for(int i=0;i<n;i++)list.add(sc.nextInt());return list; }
    static ArrayList<Long> longlist(int n){ArrayList<Long> list=new ArrayList<>();for(int i=0;i<n;i++)list.add(sc.nextLong());return list; }
    static int[][] int2darray(int n,int m){ int [][] a=new int[n][m];for(int i=0;i<n;i++){ for(int j=0;j<m;j++){ a[i][j]=sc.nextInt(); } }return a; }
    static long[][] long2darray(int n,int m){ long [][] a=new long[n][m];for(int i=0;i<n;i++){ for(int j=0;j<m;j++){ a[i][j]=sc.nextLong(); } }return a; }
    static char[][] char2darray(int n,int m){ char [][] a=new char[n][m];for(int i=0;i<n;i++){ String s=sc.next(); a[i]=s.toCharArray(); }return a; }

    static double pi=3.14159265358979323846264;
    public static double logb( double a, double b ) {return Math.log(a) / Math.log(b); }
    static long fast_pow(long a, long b,long abs) {
        if(b == 0) return 1L;
        long val = fast_pow(a, b / 2,abs);
        if(b % 2 == 0) return val * val % abs;
        else return val * val % abs * a % abs;
    }
    static long abs = (long)1e9 + 7;
    static   void shuffle(int[] a) { int n = a.length;for(int i = 0; i < n; i++) { int r = i + (int) (Math.random() * (n - i));int tmp = a[i];a[i] = a[r];a[r] = tmp; } }
    static   void shuffle(long[] a) { int n = a.length;for(int i = 0; i < n; i++) { int r = i + (int) (Math.random() * (n - i));long tmp = a[i];a[i] = a[r];a[r] = tmp; } }
    static long gcd(long a , long b) {
        if(b == 0) return a;
        return gcd(b , a % b);
    }
}

class Scanner {
    public BufferedReader reader;
    public StringTokenizer st;
    public Scanner(InputStream stream) {
        reader = new BufferedReader(new InputStreamReader(stream));
        st = null;
    }
    public String next() {
        while (st == null || !st.hasMoreTokens()) {
            try {
                String line = reader.readLine();
                if (line == null) return null;
                st = new StringTokenizer(line);
            } catch (Exception e) {
                throw (new RuntimeException());
            }
        }
        return st.nextToken();
    }
    public int nextInt() {
        return Integer.parseInt(next());
    }
    public long nextLong() {
        return Long.parseLong(next());
    }
    public double nextDouble() {
        return Double.parseDouble(next());
    }
}

Measured memory_bytes: 22612