lib C
  fun strtoll(s : UInt8*, p : UInt8**, b : Int32) : Int64
end

class String
  def to_i64
    C.strtoll(self, nil, 10)
  end
end

n = read_line.to_i
x, y = (1..n).map { read_line.split.map(&.to_i) }.transpose
puts (0...n).sum { |i| x[i - 1] * y[i] - x[i] * y[i - 1] }