895B - XK Segments - CodeForces Solution

#include <bits/stdc++.h>
using namespace std;
#pragma GCC optimize("Ofast", "unroll-loops")
const int maxn = 3e5 + 10, block = 320, mod = 1e9 + 9;
void solve(){
	long long n, x, k;
	cin >> n >> x >> k;
	long long a[n];
	for(int i = 0; i < n; i++){
		cin >> a[i];
	}
	sort(a, a + n);
	long long ans = 0;
	for(int i = 0; i < n; i++){
		long long temp = (a[i] - 1) / x;
		long long l = (temp + k) * x;
		long long r = (temp + k) * x + x - 1;
		l = max(l, a[i]);
		ans += upper_bound(a, a + n, r) - lower_bound(a, a + n, l);
	}
	cout << ans << "\n";
}
int main(){
	int t;
	t = 1;
	while(t--){
		solve();
	}
}