N,K,X,Y = map(int,input().split())
A = list(map(int,input().split()))
B = [0--(a-1)//K for a in A]
B.sort()
mem = [(0,B[0])]
for i,(a,b) in enumerate(zip(B,B[1:])):
    if a < b:
        mem.append((i+1,b))

ans = 0
p = 0
for i,a in mem:
    if Y < X*(N-i):
        ans += Y*(a-p)
    else:
        ans += X*(N-i)*(a-p)
    p = a
print(ans)