package main

import . "fmt"
import . "math/big"

func main() {
	var n string
	Scan(&n)
	bn,_:=new(Int).SetString(n,10)
	x:=new(Int).Add(bn,NewInt(1))
	x.Mul(x,bn).Div(x,NewInt(2))
	Println(x)
}