#include <bits/stdc++.h>
#define dd(x) cout << #x << "\n"
#define d(x) cout << #x  << ": " << x << "\n"
#define SZ(x) ((int)(x).size())
using namespace std;
#define int long long
using pii = pair<int, int>;
using vpii = vector<pii>;
using vi = vector<int>;
using vii = vector<vector<int>>;
using a3 = array<int, 3>;
using ll = long long;
const int inf = 0x3f3f3f3f3f3f3f3fLL;

// #define MULTI_TEST

void solve() {
    int l, r, b, k; cin >> l >> r >> b >> k;
    int a = l;
    cout << (a + b - 1) / b * b * k << "\n";
}

signed main() {
    ios::sync_with_stdio(false); cin.tie(0);
#ifdef MULTI_TEST
    int T; cin >> T;
#else
    int T = 1;
#endif
    while(T--) solve();
    return 0;
}