n = int(input())

x = [0] * n
y = [0] * n

for i in range(n):
    a, b = map(int, input().split())
    x[i] = a
    y[i] = b

area = 0
for i in range(n):
    area += x[i-1] * y[i] - x[i] * y[i-1]
print(area)