import math

class Vector:
    def __init__(self, ls: list):
        self.vec = ls

    def __len__(self):
        return len(self.vec)

    def __getitem__(self, idx):
        return self.vec[idx]

    def __repr__(self):
        return f'Vector({self.vec})'

    def add(self, vec):
        assert len(self) == len(vec)
        ret = [a+b for a, b in zip(self.vec, vec.vec)]
        return Vector(ret)

    def sub(self, vec):
        assert len(self) == len(vec)
        ret = [a-b for a, b in zip(self.vec, vec.vec)]
        return Vector(ret)

    def mul(self, vec):
        assert len(self) == len(vec)
        ret = [a*b for a, b in zip(self.vec, vec.vec)]
        return Vector(ret)

    def scalar_mul(self, x):
        ret = [a*x for a in self.vec]
        return Vector(ret)

    def scalar_div(self, x):
        ret = [a/x for a in self.vec]
        return Vector(ret)

    def norm(self):
        return math.sqrt(sum([x*x for x in self.vec]))

def dot(a, b):
    return sum(a.mul(b))

def cross(a, b):
    #outer product of 2d vector
    assert len(a) == 2 and len(b) == 2
    first = a[0]*b[1]
    second = a[1]*b[0]
    return first-second

EPS = 10**(-9)

def ccw(p0, p1, p2):
    a = p1.sub(p0)
    b = p2.sub(p0)
    if cross(a, b) > EPS:
        return 1 #'COUNTER_CLOCKWISE'
    elif cross(a, b) < -EPS:
        return -1 #'CLOCKWISE'
    elif dot(a, b) < -EPS:
        return 2 #'ONLINE_BACK'
    elif a.norm() < b.norm():
        return -2 #'ONLINE_FRONT'
    else:
        return 0 #'ON_SEGMENT'

def intersect(p0, p1, p2, p3):
    if ccw(p0, p1, p2) *ccw(p0, p1, p3) <= 0 and ccw(p2, p3, p0) *ccw(p2, p3, p1) <= 0:
        return True
    else:
        return False

def getDistanceLP(p0, p1, p):
    return abs(cross(p1.sub(p0), p.sub(p0)))/p1.sub(p0).norm()

def getDistanceSP(p0, p1, p):
    if dot(p1.sub(p0), p.sub(p0)) < 0:
        return p.sub(p0).norm()
    if dot(p0.sub(p1), p.sub(p1)) < 0:
        return p.sub(p1).norm()
    return getDistanceLP(p0, p1, p)

def area(polygon):
    s = 0
    for i in range(len(polygon)):
        s += cross(polygon[i-1], polygon[i])
    #s /= 2
    return s

n = int(input())
P = []
for i in range(n):
    x, y = map(int, input().split())
    P.append((Vector([x, y])))
s = area(P)
print(s)