n=int(input())
l=[]
for i in range(n):
    x,y=map(int,input().split())
    l.append([x,y])

ans=0
for i in range(n-2):
    ans+=abs((l[0][0]-l[i+2][0])*(l[i+1][1]-l[i+2][1])-(l[i+1][0]-l[i+2][0])*(l[0][1]-l[i+2][1]))

print(ans)