import strutils
import sequtils

var input : int
try:
  input = stdin.readLine.parseInt
except ValueError:
  echo "ValueError"
var loop:int
var num:int
#echo "input", input
if input > 0:
  num = 0
while loop < input:
 loop = loop + 1
 num = num + loop
 #echo "loop",  loop
 #echo "num",  num
echo num