n=int(input())
l=[list(map(int,input().split())) for i in range(n)]
ans=0
for i in range(1,n-1):
  x1=l[i][0]-l[0][0];y1=l[i][1]-l[0][1]
  x2=l[i+1][0]-l[0][0];y2=l[i+1][1]-l[0][1]
  ans+=abs(x1*y2-y1*x2)
print(ans)