Algorithm

input/code.cpp: In function 'int main()':
input/code.cpp:28:21: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
   for (int i = 0; i < lists[a][0].size(); ++i) {
                   ~~^~~~~~~~~~~~~~~~~~~~

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
typedef long long ll;

vector<int> lists[2][2];

int main() {
	int n;
	cin >> n;
	for (int i = 0; i < n; ++i) {
		int v;
		cin >> v;
		lists[i % 2][0].push_back(v);
	}
	for (int i = 0; i < n; ++i) {
		int v;
		cin >> v;
		lists[i % 2][1].push_back(v);
	}
	for (int a = 0; a < 2; ++a) {
		sort(lists[a][0].begin(), lists[a][0].end());
		sort(lists[a][1].begin(), lists[a][1].end(), greater<int>());
	}
	ll res = 0;
	for (int a = 0; a < 2; ++a) {
		for (int i = 0; i < lists[a][0].size(); ++i) {
			res += (ll)lists[a][0][i] * lists[a][1][i];
		}
	}
	cout << res << '\n';

	// x y z
	// a b c
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED

Test 4

Verdict: ACCEPTED

Test 5

Verdict: ACCEPTED

Test 6

Verdict: ACCEPTED

Test 7

Verdict: ACCEPTED

Test 8

Verdict: ACCEPTED

Test 9

Verdict: ACCEPTED

Test 10

Verdict: ACCEPTED

Test 11

Verdict: ACCEPTED

Test 12

Verdict: ACCEPTED

Test 13

Verdict: ACCEPTED

Test 14

Verdict: ACCEPTED

Test 15

Verdict: ACCEPTED

Test 16

Verdict: ACCEPTED

Test 17

Verdict: ACCEPTED

Test 18

Verdict: ACCEPTED

Test 19

Verdict: ACCEPTED

Test 20

Verdict: ACCEPTED