n = int(input())
xy = [list(map(int, input().split())) for i in range(n)]
xy = xy + [xy[0]]
ans = 0
for i in range(n):
    x1, y1 = xy[i]
    x2, y2 = xy[i + 1]
    ans += x1 * y2 - x2 * y1
print(abs(ans))