C++ :

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;

void solve() {
    int n;
    cin >> n;
    n *= 2;

    vector<int> a(n);
    for (int i = 0; i < n; ++i) {
        cin >> a[i];
    }

    sort(a.begin(), a.end());

    LL ans = 0;
    int l = 0, r = n - 1;
    while (l < r) {
        ans += 1LL * a[l] * a[r];
        l++, r--;
    }
    cout << ans << '\n';
}
int main() {
    ios_base ::sync_with_stdio(false);
    cin.tie(0);
    int t = 1;
    while (t--) {
        solve();
    }
    return 0;
}