#include <bits/stdc++.h>
  using namespace std;
  int main() 
  {
      int n,m,a,b; 
      cin>>n>>m>>a>>b;
      int final=0,dp[n+1];
      dp[0]=0;
      for(int i=1;i<=n;i++){
      	if(i>=m) 
 			dp[i] = min(dp[i-1]+a,dp[i-m]+b);
		else dp[i]=min(dp[i-1]+a,b);
		//cout<<dp[i]<<" ";  
      }
      cout<<dp[n];
      return 0;
 }