#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using ld = long double;

int main()
{
    ios :: sync_with_stdio(0);
    int p, k, t, r;
    cin >> p >> k >> t >> r;
    cout << ((p + k - 1) / k) * (t + r) - r;

    return 0;
}