N,K,X,Y = map(int,input().split())
lsA = list(map(int,input().split()))
lsA1 = [-(-(lsA[i]-1)//K) for i in range(N)]#必要回数
#ベホマラーの方がお得な回復人数
B = -(-Y//X)
lsA1.sort()
d = 0
ans = 0
for i in range(N):
    if lsA1[i]-d == 0:
        continue
    rm = N-i
    if rm >= B:
        ans += (lsA1[i]-d)*Y
        d = lsA1[i]
    else:
        ans += (lsA1[i]-d)*X
print(ans)