#include <fstream>

using namespace std;
int medii[7010],v[9010],i,j,n,sol,fr[9010];
int main()
{
    ifstream fi("medie.in");
    ofstream fo("medie.out");
    fi>>n;//nu numar cele care il contin pe i
    for(i=1;i<=n;i++) fi>>v[i];
    for(i=1;i<=n;i++)
    {
        for(j=i+1;j<=n;j++)
        if((v[i]+v[j])%2==0)
        {
            medii[(v[i]+v[j])/2]++;
            if((v[i]+v[j])/2==v[i]) fr[i]++;
            if((v[i]+v[j])/2==v[j]) fr[j]++;
        }
    }
    for(i=1;i<=n;i++)
    sol+=medii[v[i]]-fr[i];
    fo<<sol<<"\n";
    return 0;
}