n, k, x, y = map(int, input().split())
a = list(map(int, input().split()))

for i in range(n):
    a[i] -= 1

# 残りがcnt人以上ならベホマラーがお得
cnt = -((-y) // x)
a = sorted(a)
ind = n - cnt
if ind < 0:
    cnt_y = 0
    ans = 0
else:
    cnt_y = -((-a[ind]) // k)
    ans = cnt_y * y

for i in range(n):
    a[i] -= cnt_y * k
    if a[i] > 0:
        ans += -((-a[i]) // k) * x
print(ans)