class Scanner {
    private var stack = [String]()
    private var index = 0
    
    func peek() -> String {
        if stack.count == index {
            stack += readLine()!.split(separator: " ").map{ String($0) }
        }
        return stack[index]
    }
    
    func next() -> String {
        let value = peek()
        index += 1
        return value
    }
    
    func nextInt() -> Int {
        return Int(next())!
    }
    
    func nextDouble() -> Double {
        return Double(next())!
    }
}

extension String {
    var characters: [Character] {
        self.map{ $0 }
    }
}

var scanner = Scanner()
let N = scanner.nextInt()
let K = scanner.nextInt()
let X = scanner.nextInt()
let Y = scanner.nextInt()
var A = [Int]()
for _ in 0 ..< N {
    A.append(scanner.nextInt() - 1)
}
A.sort()

func howmany(_ a: Int, _ b: Int) -> Int {
    if a < 0 { return 0 }
    return a / b + (a % b != 0 ? 1 : 0)
}

var all = 0
var answer = 0
for i in 0 ..< N {
    let k = howmany(A[i] - K * all, K)
    if Y < (N - i) * X {
        all += k
        answer += Y * k
    } else {
        answer += X * k
    }
}

print(answer)