ABC_44 - A - 高橋君とホテルイージー / Tak and Hotels

#include <bits/stdc++.h>

#define forn(l, r) for(int i = l; i < (r); ++i)
#define forj(l, r) for(int j = l; j < (r); ++j)
#define TAM 1000100
#define SYNC ios::sync_with_stdio(0);
#define $ << ' ' <<
#define F first
#define S second
#define endl '\n'


using namespace std;

using ll = long long int;
using ii = pair<int, int>;
using vii = vector<ii>;
using vi = vector<int>;
using graph = vector<vi>;
const int INF = 0x3f3f3f3f;
const int MAXN = 112345;
const ll mod = 1000000007;




int main() {
    //SYNC
    ll ans;
    int n, k, x, y;
    cin >> n >> k >> x >> y;
    ans = min(n, k)*x;
    ans += max(0, n-k)*y;
    cout << ans << endl;
    return 0;
}