#include<fstream>
#include<iostream>
using namespace std;
ifstream f("medie.in");
ofstream g("medie.out");
int v[9005],w[7005];
int main()
{
    int i,j,N,S=0,x;
    f>>N;
    for(i=1;i<=N;i++) f>>v[i];
    for(i=1;i<=N;i++)
    {
        for(j=i+1;j<=N;j++)
        {   x=(v[i]+v[j])%2;
            if(x==0)
            {
                w[(v[i]+v[j])/2]++;
            if(v[i]==(v[i]+v[j])/2) S--;
            if(v[j]==(v[i]+v[j])/2) S--;
            }
        }
    }
    for(i=1;i<=N;i++)
    {
        S=S+w[v[i]];
    }
    g<<S;
    f.close();
    g.close();
    return 0;
}