#include <iostream>
#include <fstream>
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");
#define MAX 9004
int a[MAX];

int fr[7010*2];

int main()
{
    int n, i, j, ma;
    fin>>n;
    int s=0;
    for(i=1;i<=n;i++)
    {
        fin>>a[i];
        fr[a[i]*2]++;
    }

    for(i=1;i<=n;i++)
    {
        for(j=1+i;j<=n;j++)
        {
            s+=fr[a[i]+a[j]];
            if(a[i]==a[j])
                s-=2;
        }
    }
    fout<<s;
}