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