N = int(input())
P = [[]  for _ in range(N)]

for i in range(N):
    P[i] = list(map(int, input().split()))
    
def calc_area(p, q, r):
    if p[0] - q[0] != 0:
        m = (p[1] - q[1])/(p[0] - q[0])
        y0 = p[1] - m * p[0]
        h = abs(-m*r[0] + r[1] -y0) / pow(1 + pow(m, 2), 0.5)
    else:
        h = abs(p[0] - r[0])
    
    b = pow(pow(p[0] - q[0], 2) + pow(p[1] - q[1], 2), 0.5)
    return b * h

A = 0

#必ず凸三角形なので
for i in range(N-2):
    A += calc_area(P[i + 1], P[i + 2], P[0])

print(int(A))