1200*A. Cheap Travel

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
int n,m,a,b,res;
int main(){
	cin>>n>>m>>a>>b;
	if(a*m<=b) res+=a*n;
	else{
		if(n%m==0) res=n/m*b;
		else{
			res+=n/m*b;
			res+=min(n%m*a,b);
		}
	}
	cout<<res;
	return 0;
}