#include <algorithm>
#include <fstream>
using namespace std;
const int MAX_N = 9005;
const int MAX_VAL = 7005;

int even[MAX_N];
int odd[MAX_N];
int f[MAX_VAL];
int main() {
    ifstream fin("medie.in");
    ofstream fout("medie.out");
    int n, count_odd = 0, count_even = 0;
    fin >> n;
    for(int i = 1; i <= n; ++ i) {
        int x;
        fin >> x;
        ++ f[x];
        if(x & 1) {
            odd[++ count_odd] = x;
        } else {
            even[++ count_even] = x;
        }
    }
    int answer = 0;
    for(int i = 1; i < count_odd; ++ i) {
        for(int j = i + 1; j <= count_odd; ++ j) {
            answer += f[odd[i] + odd[j] >> 1];
            if(odd[i] == odd[j]) {
                answer -= 2;
            }
        }
    }
    for(int i = 1; i < count_even; ++ i) {
        for(int j = i + 1; j <= count_even; ++ j) {
            answer += f[even[i] + even[j] >> 1];
            if(even[i] == even[j]) {
                answer -= 2;
            }
        }
    }
    fout << answer << "\n";
    return 0;
}