#include <bits/stdc++.h>
#pragma GCC optimize ("Ofast")

using namespace std;

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

const int MAX_N = 9006;
int n, x, minn=2e9, maxx=-2e9, val[MAX_N], frq[MAX_N];
long long sol;

int main (){
    ios_base::sync_with_stdio(false);
    fin.tie(nullptr);
    fout.tie(nullptr);
    fin>>n;
    for(int i=1; i<=n; ++i){
        fin>>val[i];
        ++frq[val[i]];
        maxx = max(maxx, val[i]);
        minn = min(minn, val[i]);
    }

    for(int i=1; i<=n; i++){
        sol += (frq[val[i]]-1) * (frq[val[i]]-2) / 2;
        for(int d=1; minn <= val[i]-d && val[i]+d <= maxx; d++)
            sol += frq[val[i]-d] * frq[val[i]+d];
    }
    fout<<sol;
    return 0;
}