#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");
#define MAX 9004
int a[MAX];

int fr[7010];

int main()
{
    int n, i, j, ma;
    fin>>n;
    int s=0;
    for(i=1;i<=n;i++)
    {
        fin>>a[i];
        fr[a[i]]++;
    }

    for(i=1;i<=n;i++)
    {
        for(j=1+i;j<=n;j++)
        {
            fr[a[i]]--;
            fr[a[j]]--;

            ma=a[i]+a[j];
            if(ma%2==0)
            {
                s+=fr[ma/2];
            }

            fr[a[i]]++;
            fr[a[j]]++;

        }
    }
    fout<<s;
}