BTICK HSGTPHCM L9 2016-2017

#define task "BTICk"

#include <iostream>
#include <cstdio>

using namespace std;

using ll = long long;

ll n, k, p1, p2;

void Read()
{
    cin >> n >> k >> p1 >> p2;
}

void Solve()
{
    if (p1 >= p2)
        cout << (n + k - 1) / k * p2; // mua ceil(n / k) tệp
    else if (p1 * k <= p2)
        cout << n * p1; // Mua cả n vé lẻ
    else
    {
        ll ans = n / k * p2; // Mua n / k tệp
        ll x = n % k;        // Phần dư

        if (p1 * x >= p2)
            ans += p2;
        else
            ans += p1 * x;

        cout << ans;
    }
}

int32_t main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    if (fopen(task ".INP", "r"))
    {
        freopen(task ".INP", "r", stdin);
        freopen(task ".OUT", "w", stdout);
    }

    Read();
    Solve();
}