import sys
import numpy as np

read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

N, K, X, Y = map(int, readline().split())
A = np.array(read().split(), np.int64)

# 必要回数になおす
A -= 1
A = (A + K - 1) // K

A.sort()
A = A[::-1]
A = np.append(A, 0)

Acum = np.cumsum(A)
cntX = Acum - np.arange(1, N + 2, dtype=np.int64) * A
cntY = A

cntX = cntX.tolist()
cntY = cntY.tolist()
answer = min(x * X + y * Y for x, y in zip(cntX, cntY))
print(answer)