import java.util.*;
public class Main {
public static void main(String[] args) {
Scanner scanner = new Scanner(System.in);
int n = scanner.nextInt();
int v = scanner.nextInt();
int m = scanner.nextInt();
int a = scanner.nextInt();
int result = 0;
while(n>0){
for(int i = 1;i<=m&&n!=0;i++,n--){
result += v;
}
v += a;
}
System.out.println(result);
}
}