BOJ-3273

#include <iostream>
using namespace std;

int tmp[1000001];
int arr[100001];

int main(void)
{
	ios::sync_with_stdio(0);
	cin.tie(NULL); cout.tie(NULL);

	int n, x;
	int count = 0;
	cin >> n;

	for (int i = 0; i < n; i++)
		cin >> arr[i];
	
	cin >> x;

	for (int i = 0; i < n; i++) {
		if (tmp[x - arr[i]] == 1) count++;
		else tmp[arr[i]] = 1;
	}

	cout << count;

	return 0;
}