import java.util.Scanner
val scanner = Scanner(System.`in`)

fun sum(be:Long, la:Long): Long = (la-be+1)*(be+la)/2

fun main(){
    var n = scanner.nextLong()
    var k = scanner.nextLong()
    
    val total = sum(1,n)
    val mi = sum(1, n-k)-sum(n-k+1, n)
    
    val ans = (total-mi)/2+1
    println(ans)
}