#include <cstdio>

using namespace std;

double A[100005], B[100005];

int main() {
	freopen("aria.in", "r", stdin);
	freopen("aria.out", "w", stdout);

	int N;
	double rasp = 0;
	scanf("%d", &N);

	for(int i = 1; i <= N; ++ i)
		scanf("%lf%lf", &A[i], &B[i]);

	A[N + 1] = A[1];
	B[N + 1] = B[1];

	for(int i = 1; i <= N; ++ i)
		rasp += (A[i] * B[i + 1] - A[i + 1] * B[i]);

	printf("%.6lf\n", rasp * 0.5);

	return 0;
}