import sys

readline = sys.stdin.readline

ns = lambda: readline().rstrip()
ni = lambda: int(readline().rstrip())
nm = lambda: map(int, readline().split())
nl = lambda: list(map(int, readline().split()))


def solve():
    n, k, x, y = nm()
    a = [0] + [j-1 for j in nl()]
    a.sort()
    f = min(y // x, n)
    ans = 0
    c = a[-f-1]
    g = ((c - 1) // k + 1)
    ans = y * g + x * sum(max(j - g * k - 1, -1)//k + 1 for j in a[-f:]) * (f > 0)
    print(ans)
    return

solve()