n=int(input())
p=[]
for i in range(n):
	x,y=map(int,input().split())
	p+=[(x,y+10000)]
g=0
for i in range(n):
	sx,sy=p[i-1]
	tx,ty=p[i]
	if sx>tx:
		g+=(sy+ty)*(sx-tx)
	if tx>sx:
		g-=(ty+sy)*(tx-sx)
print(g)