40分玄关求改

#include <bits/stdc++.h>
#define int long long
using namespace std;
int n, a[100005], sum = 0;
bool book[20005];
signed main()
{
	ios::sync_with_stdio(0); 
	cin >> n;
	for(int i = 1; i <= n; i++)
	{
		cin >> a[i];
		book[a[i]] = true;
	}
	for(int i = 1; i <= n; i++)
	{
		for(int j = 1; j <= n; j++)
		{
			if(a[i] == 2 * a[j]) continue;
			if(book[a[i] - a[j]] == true)
			{
				sum++;
				break;
			}
		}
	}
	cout << sum << endl;
	return 0;
}