#include <fstream>

using namespace std;

ifstream f("medie.in");
ofstream g("medie.out");

int n,i,j;
int v[9001];
int fr1[7001];
int fr2[7001];

int main()
{
    f>>n;
    for (i=1;i<=n;i++)
    {
        f>>v[i];
        fr1[v[i]]++;
        for (j=1;j<i;j++)
            if ((v[i]+v[j])%2==0)
                fr2[(v[i]+v[j])/2]++;
    }

    long long rez=0;
    for (i=1;i<=n;i++)
        rez+=fr2[v[i]]-fr1[v[i]]+1;

    g<<rez;

    return 0;
}