N = gets.to_i
pos = Array.new(N) { gets.split.map(&:to_i) }

def polygon_area_times2(pos)
  (0 ... pos.size).sum { |i| pos[i][0] * pos[i - 1][1] - pos[i][1] * pos[i - 1][0] }.abs
end

puts polygon_area_times2(pos)