N = int(input())
XY = [list(map(int,input().split())) for _ in range(N)]
x0,y0 = XY[0]
ans = 0
for i in range(1,N-1):
    x1,y1 = XY[i]
    x2,y2 = XY[i+1]
    x1 -= x0
    y1 -= y0
    x2 -= x0
    y2 -= y0
    ans += x1 * y2 - x2 * y1
print(ans)