#include <fstream>
#include <vector>
using namespace std;
vector <int> v;
int n, i, j, q, cnt, x;
int main()
{
    ifstream f("medie.in");
    ofstream g("medie.out");
    f >> n;
    for(i = 1; i <= n; ++ i)
    {
        f >> x;
        v.push_back(x);
    }
    for(i = 0; i < n; ++ i)
        for(j=0; j<n; ++j)
            for(q=j+1; q<n; ++q)
                if(i!=j&&j!=q&&i!=q)
                    if(v[i]*2==v[j]+v[q])
                        cnt++;
    g<<cnt;
    return 0;
}