#include<stdio.h>
#include<algorithm>
using namespace std;

int V[9010],Fr[7010],n;

FILE*f=fopen("medie.in","r");
FILE*g=fopen("medie.out","w");

void read(){
	fscanf(f,"%d",&n);
	for(int i=1;i<=n;i++){
		fscanf(f,"%d",&V[i]);
		Fr[V[i]]++;
	}
}

void solve(){
	int nr=0,i,j,medie;
	for(i=2;i<=n;i++){
		medie=V[i]*2;Fr[V[i]]--;
		for(j=1;j<i;j++){
			Fr[V[j]]--;
			if(Fr[medie-V[j]]){
				nr+=Fr[medie-V[j]];
			}
			Fr[V[j]]++;
		}
		Fr[V[i]]++;
	}
	fprintf(g,"%d",nr);
}

int main(){
	read();
	sort(V+1,V+n+1);
	solve();
	return 0;
}