import algorithm, sequtils, strutils

let n = parseInt readLine stdin
var xs, ys: seq[float]

for _ in 0 ..< n:
  let
    xy = stdin.readLine.split.map parseFloat
  xs.add xy[0]
  ys.add xy[1]

let minY = min ys
if minY < 0: ys.applyIt it - minY

let
  xs2 = xs.rotatedLeft 1
  ys2 = ys.rotatedLeft 1
var sq = 0.0

for i in 0 ..< n:
  let
    x = xs[i] - xs2[i]
    y = ys[i] - ys2[i]
  if x > 0: sq += x * ys2[i] + y * x / 2
  else: sq += x * ys2[i] + y * x / 2

echo int sq * 2