#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);
    return 0;
}