#include<stdio.h>
int n,a[9100],v[7100],f[7100],nr;
void cit(){
	freopen("medie.in","r",stdin);
	scanf("%d",&n);
	for(int i=1;i<=n;i++)
		scanf("%d",&a[i]);
	fclose(stdin);
}
void solve(){
	int i,j;
	for(i=1;i<n;i++){
		f[a[i]]++;
		for(j=i+1;j<=n;j++)
			v[(a[i]+a[j])/2]++;
	}
	f[n]++;
}
int main(){
	cit();
	solve();
	freopen("medie.out","w",stdout);
	for(int i=1;i<=n;i++){
		//printf("%d ",v[i]);
		if(v[a[i]]!=0)
			nr+=v[a[i]]-f[a[i]];
	}
	printf("%d",nr);
	fclose(stdout);
	return 0;
}