# 三分探索で解いたが想定解法をやってみる
# まずHPはすべてK何回分かに書き換える
# 回復必要な残り人数kとして、k*X > yならばyベホマラー、elseベホイミ

N, K, X, Y = map(int, input().split())
A = list(map(int, input().split()))
A2 = [(a-1+(K-1))//K for a in A]
from collections import Counter
counted = Counter(A2)
counted_keys = [a for a in counted.keys()]
counted_keys.sort()

#print(A2)
#print(counted)
#print(counted_keys)

ans = 0
cum = 0
k = N
for key in counted_keys:
    if Y < k*X:
        ans += max(0, key-cum)*Y
        k -= counted[key]
        cum += key-cum
    else:
        ans += max(0, key-cum)*X
        k -= counted[key]
    #print('key', key, Y < k*X, 'ans', ans, 'k', k, 'cum', cum)
print(ans)