N, K, X, Y = map(int, input().split())
A = sorted([(int(a) - 1 + K - 1) // K for a in input().split()])
s = 0
ans = 0
for i, a in enumerate(A):
    d = a - s
    if d > 0:
        if (N - i) * X < Y:
            ans += d * X
        else:
            ans += d * Y
            s += d
print(ans)