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):
    x = [p[0] - r[0], q[0] - r[0]]
    y = [p[1] - r[1], q[1] - r[1]]
    return int(abs(x[0] * y[1] - x[1] * y[0]))

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))