//============================================================================
//Everyone has a different way of thinking, so God Created us
//   Hope You Respect My Way..,Thank You ):
// Author      : Murad
// Name        : Codeforces.cpp & Atcoder.cpp
// Description : Problem name
//============================================================================
#include<bits/stdc++.h>
#include<unordered_map>
#include<unordered_set>
using namespace std;
#define pi 3.1415926536
#define forn(i,a,b)for(int i=a;i<b;i++)
#define LL long long
#define ULL unsigned long long
#define MP make_pair
#define ff first
#define ss second
#define endl '\n'
#define INF 1e9 + 1
#define _INF INT32_MIN
#define pq priority_queue
#define MM multimap
#define PB push_back
#define vii vector<int>
#define ipair pair<int,int>
#define lpair pair<LL,LL>
#define El3zba ios::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define modd 1000000007
#define sfll1(v) scanf("%I64d",&v);
#define sfll2(v1,v2) scanf("%I64d %I64d",&v1,&v2)
#define sfll3(v1,v2,v3) scanf("%I64d %I64d %I64d",&v1,&v2,&v3)
//std::transform(s1.begin(), s1.end(), s1.begin(),::tolower);
char alphz[27] = { 'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z' };
float Euclidean(LL x1, LL x2, LL y1, LL y2) {
	return sqrt(pow(x1 - x2, 2) + pow(y1 - y2, 2));
}
LL gcd(LL a,LL b) {
	return !b ? a : gcd(b, a % b);
}
LL LCM(LL a, LL b) {
	return (a*b) / gcd(a, b);
}
void PrimeFactor(LL n)
{
	while (n % 2 == 0) {
		//printf("%d ", 2);
//		aa.insert(2);
		n /= 2;
	}
	for (int i = 3; i <= sqrt(n); i += 2)
	{
		if (n%i == 0) {
			//printf("%d ", i);
			//aa.insert(i);
			n /= i;
		}
	}
	if (n > 2){
		//printf("%d ", n);
	//	aa.insert(n);
	};
}
bool is_square(LL x) {
	LL l = 0, r = x;
	while (l <= r) {
		LL mid = l + (r - l) / 2;
		if (mid*mid == x)return true;
		if (mid*mid > x)r = mid - 1;
		else l = mid + 1;
	}
	return false;
}
LL power(LL x, LL y)
{
	LL temp;
	if (y == 0)return 1;
	temp = power(x, y / 2);
	if (y % 2 == 0)
		return temp*temp;
	else return x*temp*temp;
}
bool is_Prime(int x) {
	if (x == 2)return 1;
	else if (x % 2 == 0 || x < 2)return 0;
	for (int i = 3; i*i <= x; i += 2)
		if (x%i == 0)return 0;
	return 1;
}
const int N = 1e+5;
int aseg[4], seg[4];
void SEGBuild(int pos, int low, int hig)
{
	if (low == hig)
		seg[pos] = aseg[low];
	else
	{
		int mid = low + (hig - low) / 2;
		SEGBuild(2 * pos, low, mid);//Recurse left;;
		SEGBuild(2 * pos + 1, mid + 1, hig);// Recurse right;
		if (seg[2 * pos]<seg[2 * pos + 1])
			seg[pos] = seg[2 * pos];
		else
			seg[pos] = seg[2 * pos+1];
	}
}
void SEGUpdate(int pos,int idx,int low,int hig,int val)
{
	if (low == hig)
	{
		aseg[idx] = val;
		seg[pos] = val;
	}
	else
	{
		int mid = low + (hig - low) / 2;
		if (idx >= low&&idx <= mid)
			SEGUpdate(2 * pos, idx, low, mid, val);//Recurse left;
		else
			SEGUpdate(2 * pos + 1, idx, mid + 1, hig, val);//Recurse right;
		seg[pos]=min(seg[2*pos + 1], seg[2*pos]);
	}
}
int ANSQuery(int pos, int low, int hig, int l, int r)
{
	if (l > hig || low > r)//completely  outside;
		return 1e+5;
	if (l <= low&&r >= hig)return seg[pos];
	else {
		int p1, p2;
		int mid = low + (hig - low) / 2;
		p1 = ANSQuery(2 * pos, low, mid, l, r);//Recurse left;
		p2 = ANSQuery(2 * pos + 1, mid + 1, hig, l, r);//Recurse right;
		return min(p1, p2);
	}
}
int main()
{
	El3zba;
	/***  بسم الله الرحمن الرحيم   ***/
	int n;
	cin >> n;
	vector<int>a(n);
	for (int i = 0; i < n; i++)
		cin >> a[i];
	LL ans = 0;
	for (int i = 0; i < n; i++)
	{
		for (int j = i + 1; j < n; j++) {
			ans += a[i] * a[j];
		}
	}
	cout << ans << endl;
	return 0;
}

Measured memory_bytes: 256