N = int(input())
R = [list(map(int, input().split())) for i in range(N)]
ans = 0
for i in range(1, N - 1):
    j = i + 1
    ri = (R[i][0] - R[0][0], R[i][1] - R[0][1])
    rj = (R[j][0] - R[0][0], R[j][1] - R[0][1])
    ans += abs(ri[0] * rj[1] - ri[1] * rj[0])
print(ans)