#include<cstdio>
int x,y,v[9001],i,j,h,nr,vec[10001],s,vc[9001];
int main ()
{
freopen("medie.in","r",stdin);
freopen("medie.out","w",stdout);
int n;
scanf("%d",&n);
for(i=1;i<=n;i++)
    {
    scanf("%d",&v[i]);
    vc[v[i]]++;
    }
for(i=1;i<n;i++)
    for(j=i+1;j<=n;j++)
        if(v[i]%2==v[j]%2)
            vec[(v[i]+v[j])/2]++;
for(i=1;i<=n;i++)
    if(vc[v[i]]<3)
        {
        if(vc[v[i]]==2)
            s+=vec[v[i]]-1;
        else
            s+=vec[v[i]];
        }
    else
        s+=((vc[v[i]]-1)*(vc[v[i]]-2))/2;
printf("%d",s);
return 0;
}