#include<fstream>
#include<cstdio>
using namespace std;
FILE *f,*g;
short int i,n,j,x,y,v[9010],nr[7010];
unsigned int sol;
int main()
{
    f=fopen("medie.in","r");
    g=fopen("medie.out","w");
    fscanf(f,"%d",&n);
//    f>>n;
    for(i=1;i<=n;++i)
    {
        fscanf(f,"%d",&v[i]);
//        f>>v[i];
        ++nr[v[i]];
    }
    for(i=1;i<=n;++i)
    for(j=i+1;j<=n;++j)
    if(!((v[i]+v[j])&1))
    {
        x=(v[i]+v[j])>>1;
        if(v[i]==x)
        sol+=nr[x]-2;
        else
        sol+=nr[x];
    }
    fprintf(g,"%d\n",sol);
//    g<<sol<<'\n';
    return 0;
}