#include<fstream.h>
#include<stdio.h>
int a[10000],n,fr[10000];
long sol,v[10000];
void cit(){
	int i;
	ifstream fin("medie.in");
	fin>>n;
	for(i=1;i<=n;i++){
		fin>>a[i];
		fr[a[i]]++;
	}
	fin.close();
}
void solve(){
	int i,j;
	for(i=1;i<n;i++)
		for(j=i+1;j<=n;j++)
			if((a[i]+a[j])%2==0)
			v[(a[i]+a[j])/2]++;
	for(i=1;i<=n;i++)
		if(v[a[i]]!=0)
			sol=sol+v[a[i]]-fr[a[i]]+1;
}
int main(){
	cit();
	solve();
	//afis
	FILE *f;
	f=fopen("medie.out","w");
	fprintf(f,"%ld\n",sol);
	fclose(f);
	return 0;
}