n=int(input())
num=[list(map(int,input().split())) for _ in range(n)]
ans=0
for i in range(1,n-1):
    b=[num[i][0]-num[0][0],num[i][1]-num[0][1]]
    c=[num[i+1][0]-num[0][0],num[i+1][1]-num[0][1]]
    ans+=abs(b[0]*c[1]-b[1]*c[0])
print(ans)