n, k, x, y = map(int, input().split())
a = [0] + list(map(int, input().split()))
for i in range(1, n + 1):
    if a[i] == 1:
        a[i] = 0
    else:
        a[i] = (a[i] - 2) // k + 1
a.sort()
ans = 0
for i in range(n):
    t = a[i + 1] - a[i]
    if (n - i) * x > y:
        ans += y * t
    else:
        ans += x * t
        a[i + 1] = a[i]
print(ans)