lower_bound即可

#include <bits/stdc++.h>
#define Rep(x, a, b) for (int x = a; x <= b; x++)
#define Dep(x, a, b) for (int x = a; x >= b; x--)
#define ll long long
using namespace std;
const int MAXN = 5e5 + 5;
ll n, k, a[MAXN], ans;
int main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> n >> k;
    Rep(i, 1, n)
        cin >> a[i];
    sort (a + 1, a + n + 1);
    Rep(i, 1, n - 1)
        ans += n - (lower_bound(a + i, a + n + 1, a[i] + k) - (a + 1));
    cout << ans;
    return 0;
}