C++ :

#include <bits/stdc++.h>
using namespace std;
int main()
{
	int n;
	cin>>n;
	int a[n+1];
	for(int i=1;i<=n;i++)
		cin>>a[i];
	int k;
	cin>>k;
	sort(a+1,a+1+n);
	int ans=0;
	for(int i=1;i<=n;i++)
	{
		for(int j=1;j<=n;j++)
		{
			if(a[i]+a[j]==k)
				ans++;
			else if(a[i]+a[j]>k)
				break;
		}
	}
	cout<<ans/2<<endl;
	return 0;
}
//Author: ITProgrammer

Python :

import math
import sys
n=int(input())
s=list(map(int,input().split()))
x=int(input())
b=list()
for item in s:
    for iitem in s:
        b.append(item+iitem)
if x in b:
    print(b.count(x)//2)