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

th = int(-(-y // x))
B = [-(-(a - 1) // k) for a in A]
B.sort(reverse=True)

if th > n:
    print(sum(B))
else:
    Y_num = B[th - 1]
    ans = Y_num * y
    ans += sum([max(0, b - Y_num) * x for b in B])
    print(ans)