from sys import stdin


def main():
    input = lambda: stdin.readline()[:-1]
    N, K, X, Y = map(int, input().split())
    A = list(map(int, input().split()))

    A.sort()
    hp, mp = 0, 0
    for a in A:
        a -= hp
        if a <= 1:
            N -= 1
            continue
        n = (a + K - 1) // K
        if a - (n - 1) * K <= 1:
            n -= 1
        if N * X <= Y:
            mp += n * X
            N -= 1
        else:
            hp += n * K
            mp += n * Y
            N -= 1
    print(mp)


main()