#include<stdio.h>
FILE *f,*g;
int n,i,j,a[9001],frecv[7001],b[1][7001];
long total=0;
int main()
{
f=fopen("medie.in","r");
g=fopen("medie.out","w");
fscanf(f,"%d",&n);
for(i=1;i<=n;i++)
fscanf(f,"%d",&a[i]);
for(i=1;i<n;i++)
{
frecv[a[i]]++;
for(j=i+1;j<=n;j++)
b[(a[i]+a[j])%2][(a[i]+a[j])/2]++;
}
frecv[a[n]]++;
for(i=1;i<=n;i++)
if(b[a[i]])
total=total+b[0][a[i]]-frecv[a[i]]+1;
fprintf(g,"%ld",total);
fclose(f);
fclose(g);
return 0;
}