n, m, a, b = list(map(int, input().split()))
if b >= m*a:
print(n*a)
else:
print(min((n//m)*b + n % m*a, ((n+m-1)//m)*b))
#include<bits/stdc++.h>
using namespace std;
#define ll long long
void solve(){
int n,m,a,b;
cin>>n>>m>>a>>b;
int div=b/m;
int rem=b%m;
if(div>=a){
cout<<n*a<<endl;
}
else if(div<a){
int ans=b*(n/m)+min((n%m)*a,b);
cout<<ans<<endl;
}
}
int main(){
int t=1;
// cin>>t;
while(t--){
solve();
}
}9C - Hexadecimal's Numbers | 1265B - Beautiful Numbers |
745A - Hongcow Learns the Cyclic Shift | 873A - Chores |
1754B - Kevin and Permutation | 1547D - Co-growing Sequence |
1754D - Factorial Divisibility | 1117B - Emotes |
412B - Network Configuration | 845B - Luba And The Ticket |
1732A - Bestie | 389A - Fox and Number Game |
1732B - Ugu | 1100B - Build a Contest |
1181B - Split a Number | 1313B - Different Rules |
1736D - Equal Binary Subsequences | 1754A - Technical Support |
26B - Regular Bracket Sequence | 699A - Launch of Collider |
474D - Flowers | 1016A - Death Note |
1335C - Two Teams Composing | 1167C - News Distribution |
813C - The Tag Game | 1130C - Connect |
1236B - Alice and the List of Presents | 845C - Two TVs |
1144D - Equalize Them All | 298A - Snow Footprints |