n = int(input())
p = []
for _ in range(n):
    x, y = map(int, input().split())
    p.append((x, y))

s = 0
for i in range(n):
    s += (p[i-1][0] - p[i][0]) * (p[i-1][1] + p[i][1])

print(abs(s))