#include <bits/stdc++.h>
#define ll long long
using namespace std;
ifstream fin("medie.in");
ofstream fout("medie.out");
ll n, rez, minim = 7005, v[9005], fv[7005];

int main() {
    fin >> n;
    for (int i = 1; i <= n; ++i) {
        fin >> v[i];
        ++fv[v[i]];
    }
    for (int i = 1; i <= n; ++i) {
        rez += (fv[v[i]] - 1) * (fv[v[i]] - 2) / 2;
        for (ll st = v[i] - 1, dr = v[i] + 1; st >= 1; --st, ++dr)
            rez += fv[st] * fv[dr];
    }
    fout << rez;
    return 0;
}