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();
}
}322. Coin Change | 307. Range Sum Query - Mutable |
287. Find the Duplicate Number | 279. Perfect Squares |
275. H-Index II | 274. H-Index |
260. Single Number III | 240. Search a 2D Matrix II |
238. Product of Array Except Self | 229. Majority Element II |
222. Count Complete Tree Nodes | 215. Kth Largest Element in an Array |
198. House Robber | 153. Find Minimum in Rotated Sorted Array |
150. Evaluate Reverse Polish Notation | 144. Binary Tree Preorder Traversal |
137. Single Number II | 130. Surrounded Regions |
129. Sum Root to Leaf Numbers | 120. Triangle |
102. Binary Tree Level Order Traversal | 96. Unique Binary Search Trees |
75. Sort Colors | 74. Search a 2D Matrix |
71. Simplify Path | 62. Unique Paths |
50. Pow(x, n) | 43. Multiply Strings |
34. Find First and Last Position of Element in Sorted Array | 33. Search in Rotated Sorted Array |