n = int(input())
xy = [list(map(int,input().split())) for _ in range(n)]
ans = 0.0
for i in range(1,n-1):
    ans += abs((xy[0][0] - xy[i+1][0]) * (xy[i][1] - xy[i+1][1]) - (xy[i][0] - xy[i+1][0]) * (xy[0][1] - xy[i+1][1])) * 0.5
print(int(ans*2))