"""

"""

import sys
from sys import stdin

N = int(stdin.readline())

XY = []

for i in range(N):

    X,Y = map(int,stdin.readline().split())
    XY.append((X,Y))

ans = 0

for i in range(1,N-1):

    x0,y0 = XY[0]
    x1,y1 = XY[i]
    x2,y2 = XY[i+1]

    a1,b1 = x1-x0,y1-y0
    a2,b2 = x2-x0,y2-y0

    ans += abs(a1*b2-a2*b1)

print (ans)