def polygon_area(N, P):
    return abs(sum(P[i][0]*P[i-1][1] - P[i][1]*P[i-1][0] for i in range(N))) / 2.


n = int(input())
p = [[0 for i in range(2)] for j in range(n)]
for i in range(n):
    x, y = map(int, input().split())
    p[i][0] = x
    p[i][1] = y

print(int(polygon_area(n, p)*2))