#include <bits/stdc++.h>
using namespace std;

int main()
{
	int N;
	long long K, X, Y;
	cin >> N >> K >> X >> Y;
	vector<long long> A( N );
	for( int i = 0; i < N; i++ ) {
		cin >> A[i];
	}

	sort( A.begin(), A.end() );
	long long ans = 0;
	long long k = 1;
	while( true ) {
		int n = A.end() - upper_bound( A.begin(), A.end(), k );
		if( n == 0 ) break;
		if( n * X < Y ) ans += n * X;
		else ans += Y;
		k += K;
	}

	cout << ans << endl;
}