784 B | 742 chars
import sys
from math import sqrt, pi, sin, factorial, ceil, floor
from datetime import datetime, timedelta
# sys.setrecursionlimit(10**7)
BLANK = " "
# inp = input
inp = lambda : sys.stdin.readline().rstrip()
mii = lambda x = BLANK : [*map(int,inp().split(x))]
mfi = lambda x = BLANK : [*map(float,inp().split(x))]
ii = lambda : int(inp())
fi = lambda : float(inp())
p = print
def gcd(a, b): return gcd(b, a % b) if b > 0 else a
def lcm(a, b): return a * b // gcd(a, b)
def solve():
left, right, b, k = mii()
crt = left
# l ~ r 구간
# b 마다 등장
# 한번의 쿨타임 동안 얼마나 놓치는가.?
last_duck = left // b + int(-1 if left % b == 0 else 0)
p(k * (last_duck + 1) * b)
if __name__ == "__main__":
tc = 1
for t in range(1, tc+1):
ret = solve()