#include <fstream>
#define NMAX 9001

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

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