Baekjoon #14929

#include<bits/stdc++.h>

using namespace std;
typedef long long ll;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);

    int N; cin >> N;
    ll sum = 0, square_sum = 0;
    for(int i=0;i<N;i++) {
        ll x; cin >> x;
        sum += x;
        square_sum += x * x;
    }
    cout << (sum * sum - square_sum) / 2;

    return 0;
}

import java.util.*;
import java.lang.*;
import java.io.*;

public class Main {
    public static void main(String[] args) {
        FastReader rd = new FastReader();
        int N = rd.nextInt();

        int[] O = new int[N + 1];
        int[] A = new int[N + 1];

        for(int i=1; i<=N; i++){
            O[i] = rd.nextInt();
            A[i] = A[i-1] + O[i];
        }

        long sum = 0;
        for(int i=1; i<=N; i++){
            sum += A[i-1]*O[i];
        }

        System.out.println(sum);
    }

    static class FastReader {
        BufferedReader br;
        StringTokenizer st;

        public FastReader() {
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        String next() {
            while(st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                }
                catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int nextInt() { return Integer.parseInt(next()); }
        long nextLong() { return Long.parseLong(next()); }
        double nextDouble() { return Double.parseDouble(next()); }
        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            }
            catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }
    }
}