# 入力値を数値で返す
def gets_i()
  gets.chomp.to_i
end

N = gets_i

# n_list = (1..N).to_a
# p_list = (2..N).to_a + [1]
#
# sum = 0
# n_list.each_with_index do |n, i|
#   sum += n % p_list[i]
# end
# puts sum

puts N * (N-1) / 2
