N, K, X, Y = map(int, input().split())
A = list(map(int, input().split()))
A.sort()

ans = 0
cnt = 0
for i, a in enumerate(A):
    a -= 1
    a = max(0, a - cnt * K)
    if a == 0:
        continue
    spell = (a + K - 1) // K
    ans += min(Y, (N - i) * X) * spell
    cnt += spell

print(ans)