#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");
int j,i,ma,N,ap[7005],x[9005];
long long cont;
int main()
{
    fin>>N;
    for(i=1;i<=N;i++)
    {
        fin>>x[i];
        ap[x[i]]++;
    }
    for(i=1;i<N;i++)
    {
        for(j=i+1;j<=N;j++)
        {
            if((x[i]+x[j])%2==0)
            {
                ma=(x[i]+x[j])/2;
                if(ma!=x[i] && ma!=x[j] && ap[ma]>0)
                {
                    cont+=ap[ma];
                }
                if(ma==x[i] && ap[ma]>2)
                {
                    cont+=ap[ma]-2;
                }
            }
        }
    }
    fout<<cont<<"\n";
    return 0;
}