human
Ann has recently started commuting by subway. We know that a one ride subway ticket costs a rubles. Besides, Ann found out that she can buy a special ticket for m rides (she can buy it several times). It costs b rubles. Ann did the math.
she will need to use subway n times. Help Ann, tell her what is the minimum sum of money she will have to spend to make n rides?

Input

The single line contains four space-separated integers n, m, a, b (1 ≤ n, m, a, b ≤ 1000) — the number of rides Ann has planned, the number of rides covered by the m ride ticket, the price of a one ride ticket and the price of an m ride ticket. 

Output

Print a single integer — the minimum sum in rubles that Ann will need to spend.

Examples

Input

6 2 1 2

Output

6

Input

5 2 2 3

Output

8

Note

In the first sample one of the optimal solutions is: each time buy a one ride ticket. There are other optimal solutions. For example, buy three m ride tickets.
Using python3 can you solve the prior task?
 Write me a Python program to print a single integer — the minimum sum in rubles that Ann will need to spend?
assistant
#Here is Python program to print a single integer — the minimum sum in rubles that Ann will need to spend.

"""
 
n=int(z())
 
for _ in range(int(z())):
 
x=int(z())
 
l=list(map(int,z().split()))
 
n=int(z())
 
l=sorted(list(map(int,z().split())))[::-1]
 
a,b=map(int,z().split())
 
l=set(map(int,z().split()))
 
led=(6,2,5,5,4,5,6,3,7,6)
 
vowel={'a':0,'e':0,'i':0,'o':0,'u':0}
 
color4=["G", "GB", "YGB", "YGBI", "OYGBI" ,"OYGBIV",'ROYGBIV' ]
 
"""
#!/usr/bin/env python
#pyrival orz
import os
import sys
from io import BytesIO, IOBase

input = sys.stdin.readline
############ ---- Input Functions ---- ############
def inp():
    return(int(input()))
def inlt():
    return(list(map(int,input().split())))
def insr():
    s = input()
    return(list(s[:len(s) - 1]))
def invr():
    return(map(int,input().split()))
from math import ceil
def main():
    # Solution Here
    
    n,m,a,b = invr()
    div, rem = divmod(n, m)
    print(min(a*n, div*b + min(a*(rem), b)))

        

# region fastio

BUFSIZE = 8192

class FastIO(IOBase):
    newlines = 0

    def __init__(self, file):
        self._fd = file.fileno()
        self.buffer = BytesIO()
        self.writable = "x" in file.mode or "r" not in file.mode
        self.write = self.buffer.write if self.writable else None

    def read(self):
        while True:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            if not b:
                break
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines = 0
        return self.buffer.read()

    def readline(self):
        while self.newlines == 0:
            b = os.read(self._fd, max(os.fstat(self._fd).st_size, BUFSIZE))
            self.newlines = b.count(b"\n") + (not b)
            ptr = self.buffer.tell()
            self.buffer.seek(0, 2), self.buffer.write(b), self.buffer.seek(ptr)
        self.newlines -= 1
        return self.buffer.readline()

    def flush(self):
        if self.writable:
            os.write(self._fd, self.buffer.getvalue())
            self.buffer.truncate(0),...