s = Array.new(gets.to_i) { |i|
  i.even? ? 'a' : 'b'
}

a_num = (s[2..-1] || []).count('a')
b_num = (s[3..-1] || []).count('b')

puts 1.upto(a_num).inject(:+) + 1.upto(b_num).inject(:+)