#include<fstream>
#include<iostream>
#include<algorithm>
#include<cstring>
#define DN 9005
#include<deque>
#define x first
#define y second
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");
int n,a[DN],fr[DN],f;
long long rez;
int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
    {
        fin>>a[i];
        fr[a[i]]++;
    }
    for(int i=1;i<=n;i++)
        for(int j=i+1;j<=n;j++)
            if(a[i]%2==a[j]%2)
            {
                f=(a[i]+a[j])/2;
                if(f==a[i])
                    rez+=fr[f]-2;
                else
                    rez+=fr[f];
            }
    fout<<rez;
}