#include<stdio.h>
FILE *f,*g;
int main () {
	long long nr,n,i,j;
	long a[20000],v[20000],x[20000];
	f=fopen("medie.in","r");
	g=fopen("medie.out","w");
	fscanf(f,"%ld\n",&n);
	for (i=1;i<=n;i++) {
		fscanf(f,"%ld\n",&a[i]);
		x[a[i]]++;
	}
	for (i=1;i<=n-1;i++)
		for (j=i+1;j<=n;j++)
			v[a[i]+a[j]]++;
	for (i=1;i<=n;i++)
		nr=nr+v[2*a[i]];
	for (i=1;i<=n;i++)
		nr=nr-5x[a[i]]+1;
	fprintf(g,"%ld ",nr);
	fclose(f);
	fclose(g);
	return 0;
}