#include <stdio.h>
int i,j,n,nrtot,max;
int a[9010],nr[7010],fol[14010];

int main()
{

    
    freopen("medie.in","r",stdin);
    freopen("medie.out","w",stdout);
    
    scanf("%d",&n);
    for (i=1; i<=n; i++)
    {
        scanf("%d",&a[i]);
        if (a[i]>max) max=a[i];
        nr[a[i]]++;
    }
    
    for (i=1; i<=n-1; i++)
        for (j=i+1; j<=n; j++)
        if ((a[i]+a[j])%2==0)
           fol[(a[i]+a[j])/2]++;                     

    nrtot=0;
    for (i=1; i<=max; i++)
        if (fol[i]!=0 && nr[i]!=0) nrtot+=nr[i]*fol[i]-nr[i]*(nr[i]-1);       
            
    
    printf("%d\n",nrtot);
    
    
    return 0;    
}