#include <fstream>
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");
int n,x,i,j,sol,v[9001],f[7001];
int main() {
    fin>>n;
    for (i=1;i<=n;i++) {
        fin>>v[i];
        f[v[i]]++;
    }
    sol=0;
    for (i=1;i<n;i++)
        for (j=i+1;j<=n;j++)
            if ((v[i]+v[j])%2==0) {
                sol+=f[(v[i]+v[j])/2];
                if (v[i]==v[j])
                    sol-=2;
            }
    fout<<sol;
    return 0;
}