def cross_product(X):
    x0, y0, x1, y1 = X
    return x0 * y1 - y0 * x1

def calcdif(X):
    x0, y0, x1, y1 = X
    return [x0 - x1, y0 - y1]

N = int(input())
X = [list(map(int,input().split())) for _ in range(N)]
ans = 0
for i in range(1,N-1):
    ans += abs(cross_product(calcdif(X[i] + X[0])+ calcdif(X[i+1]+X[0])))
print(ans)