#include<stdio.h>
int n,s,a[9001],r[7001],v[7001],i,j,x;
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),r[a[i]]++;
    for(i=1;i<7001;i++)
    for(j=0;j<r[i];j++)
        a[++x]=i;
    for(i=1;i<n;i++)
    for(j=i+1;j<=n;j++)
    if(!((x=a[i]+a[j])&1))
        ++v[x>>1];
    for(i=1;i<=n;i++)
    if(v[a[i]])
        s+=v[a[i]]-r[a[i]]+1;
    printf("%d",s);
}