#ifdef MY_LOCAL
#include "D://competitive_programming/debug/debug.h"
#define debug(x) cerr << "[" << #x<< "]:"<<x<<"\n"
#else
#define debug(x) 
#endif
#define REP(i, n) for(int i = 0; i < (n); i ++)
#define REPL(i,m, n) for(int i = (m); i < (n); i ++)
#define SORT(arr) sort(arr.begin(), arr.end())
#define LSOne(S) ((S)&-(S))
#define sz(X) ((int)X.size())
#define READ(arr) for(auto &a: arr){cin>>a;}
#define SUM(arr) accumulate((arr).begin(), (arr).end(), 0LL)
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>
using namespace __gnu_pbds;
using namespace std;
typedef long long ll;
#define int ll
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int, int> ii;
typedef vector<ii> vii;
typedef vector<vii> vvii;
typedef tree<int,null_type,less<int>, rb_tree_tag, tree_order_statistics_node_update> ost;
const ll INF = 1e18;

signed main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);
	int l,r,b,k;cin>>l>>r>>b>>k;
	int mult = l/b;
	if (l%b != 0) {
		mult++;
	}
	cout<<mult*b*k;
}