# include <cassert>
# include <cstdio>
# include <cmath>

const char *FIN = "aria.in", *FOU = "aria.out";
const int MAX = 100005;

struct punct {
	double x, y;
} vec[MAX];

int N;
double sol;

int main(void) {
	freopen(FIN, "r", stdin);
	freopen(FOU, "w", stdout);

	scanf("%d", &N);
	for (int i = 0; i < N; ++i) {
		scanf("%lf %lf", &vec[i].x, &vec[i].y);
	}
	vec[N] = vec[0];
	for (int i = 0; i < N; ++i)
		sol += (vec[i].x * vec[i + 1].y - vec[i + 1].x * vec[i].y);
	printf("%lf", fabs(sol / 2.0));
}