#include <fstream>
using namespace std;

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

const int N_MAX = 9005;
const int VAL_MAX = 7005;

int N;
int a[N_MAX];
int cnt[VAL_MAX], f[VAL_MAX];
int64_t sol;

void read() {
    in >> N;
    for (int i = 1; i <= N; ++i)
        in >> a[i];
}

void solve() {
    for (int i = 1; i <= N; ++i) {
        ++f[a[i]];
        
        for (int j = i + 1; j <= N; ++j)
            if ((a[i] + a[j]) % 2 == 0)
                ++cnt[(a[i] + a[j]) / 2];
    }
    
    for (int i = 1; i <= N; ++i)
        if (cnt[a[i]])
            sol += cnt[a[i]] - f[a[i]] + 1;
    
    out << sol;
}

int main() {
    read();
    solve();
}