n = int(input())
x,y = [0]*n,[0]*n
for i in range(n):
	x[i],y[i] = map(int,input().split())
ans = 0
for i in range(1,n-1):
	a = [x[i]-x[0],y[i]-y[0]]
	b = [x[i+1]-x[0],y[i+1]-y[0]]
	ans += abs(a[0]*b[1]-a[1]*b[0])
print(ans)