#include <cstdio>
#include <utility>
using namespace std;

using punct = pair<double, double>;

double det(const punct& a, const punct& b){
	return (a.first * b.second - a.second * b.first)/2; }

int main(){
	FILE *f = fopen("aria.in", "r"),
		*g = fopen("aria.out", "w");
	int n;
	fscanf(f, "%d ", &n);
	punct primul, penultimul, ultimul;
	fscanf(f, "%lf %lf ", &primul.first, &primul.second);
	ultimul = primul;
	double rez = 0;
	for(int i = 1; i < n; ++i){
		penultimul = ultimul;
		fscanf(f, "%lf %lf ", &ultimul.first, &ultimul.second);
		rez += det(penultimul, ultimul); }
	rez += det(ultimul, primul);
	fprintf(g, "%lf", rez);
	fclose(f), fclose(g);
	return 0; }