#include <fstream>
#define IN "aria.in"
#define OUT "aria.out"
#define NMAX 100005

using namespace std;

ifstream in(IN);
ofstream out(OUT);

struct PUNCT{int x, y;}v[NMAX];

inline int Area(int x1, int x2, int y1, int y2)
{
	return (x1*y2)-(x2*y1);
}

int main()
{
	int i, n;
	float area=0;
	in>>n;
	for(i=1; i<=n; ++i)
		in>>v[i].x>>v[i].y;
	for(i=1; i<n; ++i)
		area+=Area(v[i].x, v[i+1].x, v[i].y, v[i+1].y);
	area+=Area(v[n].x, v[1].x, v[n].y, v[1].y);
	area/=2;
	out<<area<<'\n';
	in.close();
	out.close();
	return 0;
}