N = int(input())
XY = [list(map(int, input().split())) for _ in range(N)]
x, y = XY[0]
S = 0
for i in range(1, N-1):
    ax, ay = XY[i]
    bx, by = XY[i+1]
    ax-=x
    ay-=y
    bx-=x
    by-=y
    S += abs(ax*by-ay*bx)
print(S)