n = int(input())
x1, y1 = list(map(int, input().split()))
sx, sy = x1, y1

ans = 0
for i in range(n-1):
  x2, y2 = list(map(int, input().split()))
  ans += x1*y2 - x2*y1
  x1, y1 = x2, y2
ans += x1*sy - sx*y1
print(ans)