package Yukicoder

/**
 * Created by hichikawa on 2015/11/12.
 */
fun main(args: Array<String>) {
    fun readLineIntArray() : List<Int> {
        val str = readLine() as String
        val arrStr = str.split(" ")
        val ret = arrStr.map { it.toInt() }
        return ret
    }

    val inputArrInt = readLineIntArray()
    val val1 = inputArrInt[0] * inputArrInt[3]
    val val2 = inputArrInt[1] * inputArrInt[2]
    if (val1 <= val2) {
        val ans = inputArrInt[3] + (inputArrInt[3] * (inputArrInt[0].toDouble() / inputArrInt[1]))
        println(ans)
    } else {
        val ans = inputArrInt[2] + (inputArrInt[2] * (inputArrInt[1].toDouble() / inputArrInt[0]))
        println(ans)
    }
}