#include<cstdio>
#include<complex>
#include<cmath>
using namespace std;
typedef complex<int> xy;


int main() {
	xy p[110];
	int n;
	scanf("%d", &n);
	int x, y;

	for (int i = 0; i < n; i++) {
		scanf("%d %d", &x, &y);
		p[i] = xy(x,y);
	}

	double sum = 0.0;
	for (int i = 1; i <= n - 2; i++) {
		xy a = p[0], b = p[i], c = p[i + 1];
		sum += (conj(b - a)*(c - a)).imag() / 2.0;
	}
	printf("%.1f\n", abs(sum));

	return 0;
}