import math

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

cnt = math.ceil(Y / X)#ベホマラーのほうが良い人数
if cnt >= N:
    print(sum(C) * X)
    exit()
else:
    B = C[-cnt]
    #print("A = ",A)
    #print("cnt = ", cnt)
    #print("B = ",B)
    #print("C = ",C)
    #print(C[-cnt:])
    ans = 0
    ans += B * Y
    tmp = sum(C[-cnt:]) - B *cnt
    ans += tmp * X
    print(ans)