import kotlin.math.*

fun main(args: Array<String>) {
    val (a, b, c, d) = readLine()!!.split(" ").map(String::toInt)
    // val red = (a..b).toList()
    // val black = (c..d).toList()
    val all = (b - a + 1) * (d - c + 1) 
    val same = max(0, min(b, d) - max(a, c) + 1)
    println(all - same)
}