N = gets.to_i
P = N.times.map { gets.split.map(&:to_i) }

x1, y1 = P[0]
ans = 0

1.upto(N - 2) do |i|
  x2, y2 = P[i]
  x3, y3 = P[i + 1]

  ans += ((x1 - x3) * (y2 - y3) - (x2 - x3) * (y1 - y3)).abs
end

puts ans