#include <iostream> 
using namespace std;
int n,a[105],b[10005],num=1,tot;
int main(){
	cin>>n;
	for(int i=1;i<=n;++i) cin>>a[i];
	for(int i=1;i<=n-1;++i){
		for(int j=i+1;j<=n;++j){
			b[num] = a[i]+a[j];
			num++;
		}
	} 
	for (int i = 1; i <= num - 2; ++i) {
		for (int j = 1; j <= num - i+1; ++j) {
			if (b[j] > b[j + 1]) swap(b[j], b[j + 1]);
		}
	}
	for(int i=1;i<=num-1;++i){
		if(b[i]=b[i+1]) b[i]= 99999;
	}
	for(int i=1;i<=n;++i){
		for(int j=1;j<=num-1;++j){
			if(a[i]==b[j]) ++tot;
		}
	}
	cout<<tot;
	return 0;
}