n = int(input())
a = [list(map(int, input().split())) for i in range(n)]
ans = 0
for i in range(1, n-1):
  ans += abs((a[0][0]-a[i+1][0])*(a[i][1]-a[i+1][1])-(a[i][0]-a[i+1][0])*(a[0][1]-a[i+1][1]))
print(ans)