#include <fstream>
#include <climits>
#include <algorithm>

using namespace std;

ifstream fin("medie.in");
ofstream fout("medie.out");

int n,ans,i,j;

int a[9000],v[7001];

int main()
{
    fin>>n;
    for(i=0;i<n;i++)
    {
        fin>>a[i];
        v[a[i]]++;
    }
    for(i=0;i<n;i++)
        for(j=i+1;j<n;j++)
            if((a[i]+a[j])%2==0)
            {
                ans+=v[(a[i]+a[j])/2];
                if((a[i]+a[j])/2==a[i])ans--;
                if((a[i]+a[j])/2==a[j])ans--;
            }
    fout<<ans<<'\n';
    return 0;
}