https://www.luogu.com.cn/problem/U496299

#include <bits/stdc++.h>
using namespace std;
int a[200005];
int cnt;
int main() {
    //freopen("elements.in","r",stdin);
	//freopen("elements.out","w",stdout);
	int n;
	int m;
	cin>>n;
	for(int i=1;i<=n;i++){
		scanf("%d",&a[i]);
	}
	cin>>m;
	for(int i=1;i<=n-1;i++){
		for(int j=i+1;j<=n;j++){
			if(a[j]+a[i]==m) cnt++;
		}
	}
	cout<<cnt;
    return 0;
}