N, K, X, Y = map(int, input().split())
A = list(map(int, input().split()))
A.sort()
for i in range(N):
    A[i] -= 1

m = 0
for i in range(N + 1):
    if i * X < Y:
        m = i
ans = 0
if m == N:
    for i in range(N):
        ans += (A[i] + K - 1)//K * X
    print(ans)
    exit()
m = N - m - 1
cnt = (A[m] + K - 1)//K
ans = cnt * Y
for i in range(N):
    A[i] = max(0, A[i] - cnt * K)
    ans += (A[i] + K - 1)//K * X
print(ans)