#include<bits/stdc++.h>
using namespace std;
int n,ans=0;
int s[101];
int d[20001];
int main(){
	
cin>>n;
for(int i=1;i<=n;i++){
	cin>>s[i];
}
	for(int i=1;i<=n;i++){
	for(int j=i+1;j<=n;j++){
d[s[i]+s[j]]++;
}
}
sort(s+1,s+n+1);//s={1,2,3,4}
//for(int i=1;i<=n;i++){
//	cout<<s[i]<<" ";
//}
for(int i=1;i<=n;i++){
	if(d[s[i]]!=0){
	ans++;
//	cout<<d[s[i]]<<" ";
	} 
}
cout<<ans;
}

/**************************************************************
	Problem: 2330
	User: chenyubo
	Language: C++
	Result: Accepted
	Time:47 ms
	Memory:2156 kb
****************************************************************/