#yuki1447
n=int(input())
l=[]
for i in range(n):
 x,y=map(int,input().split())
 l.append((x,y))
res=0
for i in range(n):
 res+=(l[i-1][0]-l[i][0])*(l[i-1][1]+l[i][1])
print(abs(res))