import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines
sys.setrecursionlimit(10 ** 7)

n, k, x, y = map(int, readline().split())
a = list(map(int, readline().split()))
a.sort()
ans = 0
now = 1
for i, aa in enumerate(a):
    if aa - now <= 0:
        continue
    m = (aa - now) // k
    ans += min(m * y, m * x * (n - i))
    now += m * k
    if (aa - now) % k != 0:
        ans += min(x * (n - i), y)
        now += k
print(ans)