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

# 平行四辺形の面積を考える
ans = 0
sx, sy = P[0]
for i in range(1, n - 1):
    ax, ay = P[i]
    bx, by = P[i + 1]
    ux, uy = ax - sx, ay - sy
    vx, vy = bx - sx, by - sy
    ans += abs(ux * vy - uy * vx)
print(ans)