#include <fstream>

using namespace std;

int n, val;

int v[9005], ap[9005];

long long sol;

int main() {
    ifstream f("medie.in");
    ofstream g("medie.out");
    f>>n;
    sol = 0;
    for(int i=1; i<=n; ++i) {
        f>>v[i];
        for(int j=1; j<i; ++j) {
            if((v[i]+v[j])%2 == 0) {
                val=(v[i]+v[j])/2;
                ap[val]++;
                if(val == v[i])
                    sol-=2;
            }
        }
    }
    for(int i=1; i<=n; ++i)
        sol += ap[v[i]];
    g<<sol<<"\n";
    return 0;
}