#include<stdio.h>
#include<stdlib.h>
#define NMAX 9004
#define FIN "medie.in"
#define FOUT "medie.out"
#include<algorithm>
using namespace std;
int n,a[NMAX],rep[NMAX],v[NMAX];
int main()
{freopen(FIN,"r",stdin);
freopen(FOUT,"w",stdout);
scanf("%d",&n);
for(int i=0;i<n;i++)
 scanf("%d",&a[i]); 
sort(a,a+n); 
for(int i=0;i<n-1;i++)
 {++rep[a[i]];
  for(int j=i+1;j<n;j++)
   {if(!((a[i]+a[j])&1)) 
     ++v[((a[i]+a[j])>>1)];
          }
        }
++rep[a[n-1]];                
int contor=0;
for(int i=0;i<n;i++)
 {if(v[a[i]])
   contor+=v[a[i]]+1-rep[a[i]]; }          
printf("%d",contor);        
    return 0;}