import os

N=int(input())

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

b = 0
for i in range(N):
    x_0, y_0 = l[i] 
    x_1, y_1 = l[(i+1)%N]
    
    b += x_0*y_1 - x_1*y_0

print(b)