N, K, X, Y = map(int,input().split())
ls = list(map(int,input().split()))
ls.sort(reverse = True)

for i in range(N):
    ls[i] = - ((-ls[i] + 1) // K)
    
a = Y // X #a以下はベホイミ

if a > N:
    print(sum(ls) * X)
    exit()

b = ls[a]
ans = Y * b    

for i in range(N):
    if ls[i] > b:
      ans += (ls[i] - b) * X 
    else:
        break
    
print(ans)