#include <iostream>
#include <fstream>
#include <algorithm>
using namespace std;

ifstream in("medie.in");
ofstream out("medie.out");
int v[9002], frecv[7001], n, i, j, caut, st, dr, mij;
long long sol;
int main()
{
    in >> n;
    for(i = 1; i <= n; i++) in >> v[i], frecv[v[i]]++;

    //sort(v+1, v+n+1);

    for(i = 1; i <= n; i++) {
        for(j = i+1; j <= n; j++) {
            caut = v[i]+v[j];
            if(caut%2 != 0) continue;
            else {
                caut = caut/2;
                sol += frecv[caut];
                if(v[i] == caut) sol--;
                if(v[j] == caut) sol--;
            }
            /*else {
                caut = caut/2;
                st = i, dr = j;
                while(st < dr) {
                    mij = st + (dr-st)/2;
                    if(v[mij] == caut) sol++, st = dr;
                    else if(v[mij] < caut) st = mij+1;
                    else if(v[mij] > caut) dr = mij;
                }
            }*/
        }
    }
    out << sol;
    return 0;
}