text: #include <iostream>
  #include <math.h>
   
  using namespace std;
   
  int main()
  {
      int n, m, a, b;
      
      cin >> n >> m >> a >> b;
      if (m * a <= b)
          cout << n * a << "\n";
      else 
          cout << (n/m) * b + min((n%m) * a, b) << "\n";
   
      return 0;
  }