n=int(input())
point=[list(map(int,input().split())) for _ in range(n)]
s=0
sx,sy=point[0][0],point[0][1]
for i in range(n-1):
    x,y=point[i]
    X,Y=point[i+1]
    s+=abs((x-sx)*(Y-sy)-(y-sy)*(X-sx))
print(s)