local n = 1ULL * io.read("*n")
if n % 2ULL == 1ULL then
  a = (n + 1ULL) / 2ULL
  a = a * n
  a = tostring(a):gsub("ULL", "")
  print(a)
else
  a = n / 2ULL
  a = a * (n + 1ULL)
  a = tostring(a):gsub("ULL", "")
  print(a)
end