N = int(input())
X, Y = [0] * N, [0] * N
for i in range(N):
	X[i], Y[i] = map(int, input().split())
	if i:
		X[i] -= X[0]
		Y[i] -= Y[0]
	
ans = 0
for i in range(1, N-1):
	ans += abs(X[i] * Y[(i + 1) % N] - X[(i + 1) % N] * Y[i])
print(ans)