n = int(input())
xy = [list(map(int, input().split())) for _ in range(n)]

ans = 0
x0, y0 = xy[0]
for i in range(1, n - 1):
    x1, y1 = xy[i]
    x2, y2 = xy[i + 1]
    dx1 = x1 - x0
    dy1 = y1 - y0
    dx2 = x2 - x0
    dy2 = y2 - y0
    ans += abs(dx1 * dy2 - dy1 * dx2)
print(ans)