#include<cstdio>
#include<algorithm>
using namespace std;
#define N 9001
#define X 7001
int n,s,a[N],r[X],v[X],i,j,x;
int main()
{FILE *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]),r[a[i]]++;
sort(a+1,a+n+1);
for(i=1;i<n;i++)
for(j=i+1;j<=n;j++)
if((x=a[i]+a[j])%2==0)
     ++v[x/2];
for(i=1;i<=n;i++)
if(v[a[i]])
     s+=v[a[i]]-r[a[i]]+1;
fprintf(g,"%d",s);
return 0;}