#include<fstream.h>
ifstream f("medie.in");
ofstream g("medie.out");
int n,i,j,a[9001],frecv[7001],b[7001];
long long total=0;
int main()
{
f>>n;
for(i=1;i<=n;i++)
f>>a[i];
for(i=1;i<n;i++)
{
frecv[a[i]]++;
for(j=i+1;j<=n;j++)
if((a[i]+a[j])%2==0)
b[(a[i]+a[j])/2]++;
}
frecv[a[n]]++;
for(i=1;i<=n;i++)
if(b[a[i]])
total=total+b[a[i]]-frecv[a[i]]+1;
g<<total;
f.close();
g.close();
return 0;
}