#include<bits/stdc++.h>
using namespace std;
ifstream fin ("medie.in");
ofstream fout("medie.out");
int v[9004],fv[7005],suma[14005];

int main ()
{
    int n,i,j,sol=0;
    fin>>n;
    for (i=1;i<=n;i++)
        {
            fin>>v[i];
            fv[v[i]]++;
        }
    for (i=1;i<=n;i++)
        for (j=i+1;j<=n;j++)
            suma[v[i]+v[j]]++;
    for (i=1;i<=n;i++)
        if (suma[2*v[i]]!=0)
            sol=sol+suma[2*v[i]]-(fv[v[i]]-1);
    fout<<sol<<'\n';
}