#include <bits/stdc++.h>
using namespace std;

ifstream ci("aria.in");
ofstream cou("aria.out");

struct punct
{
	long double x,y;
};

long double S;
int n;
punct v[100005];

void citire()
{
	ci>>n;

	for(int i=1; i<=n; i++)
	{
		ci>>v[i].x>>v[i].y;
	}
	v[n+1]=v[1];
	for(int i=1; i<=n; i++)
	{
		S+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
	}


	cou<<fixed<<setprecision(6)<<(long double)S/2;
}


int main()
{
	citire();

	return 0;
}