Count the number of pairs where i < j using C++

#include <bits/stdc++.h>

using namespace std;

int main() {
    int n;
    cin >> n;
    int a[n];
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    cout << (n * (n - 1)) / 2 << '\n';
    return 0;
}