csp 相反數

#include<iostream>
using namespace std;
int n;
int a[505];
int vis[505];
int ans = 0;
int main()
{
	cin >> n;
	for (int i = 0; i < n; i++)
	{
		cin >> a[i];
	}
	for (int i = 0; i < n; i++)
	{
		if (vis[i] == 1)
		{
			continue;
		}
		for (int j = 0; j < n; j++)
		{
			if (vis[j] == 0)
			{
				if (a[i] == -a[j])
				{
					ans += 1;
					vis[j] = 1;
				}
			}
		}
	}
	cout << ans;
	return 0;
}