/*#include <iostream>
#include <fstream>
#include <iomanip>
#include <cmath>

using namespace std;

ifstream fi ("aria.in");
ofstream fo ("aria.out");

struct point {
    long long x, y;
};

point P[100001];
int n;
double AREA;

double triangle_area (point A, point B, point C)
{
    A.x -= C.x;
    B.x -= C.x;
    A.y -= C.y;
    B.y -= C.y;
    return fabs (double ((A.x * B.y - B.x * A.y) / 2));
}

int main()
{
    fi >> n;
    for (int i = 1; i <= n; ++ i)
        fi >> P[i].x >> P[i].y;
    for (int i = 2; i <= n - 1; ++ i)
        AREA += triangle_area (P[1], P[i], P[i + 1]);
    fo << AREA;
    fi.close ();
    fo.close ();
    return 0;
}*/
#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

#define NMax 100010

struct Punct{
	long double x, y;
};

typedef Punct Poligon[NMax];

int n;
Poligon P;

void citire();
long double aria(Poligon &);
long double abs(long double x) {return (x < 0) ? -x : x;}

int main(){
	citire();
	fout << fixed << setprecision(5) << aria(P);
}

void citire(){
	int i;
	fin >> n;
	for(i = 1; i <= n; i++) fin >> P[i].x >> P[i].y;
	P[0] = P[n];
	P[n + 1] = P[1];
}

long double aria(Poligon & P){
	int i;
	long double a = 0;
	for(i = 1; i <= n; i++)
		a += P[i].x * (P[i + 1].y - P[i - 1].y);
	return abs(a) / 2;
}