n = int(input())
xy = [list(map(int,input().split())) for i in range(n)]
ans = 0
x,y = xy[0]
for i in range(n):
    a,b = xy[i]
    xy[i] = a-x,b-y

for (a,b),(c,d) in zip(xy[1:],xy[2:]):
    ans += abs(a*d-b*c)
print(ans)