#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

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

#define limn 100010
struct {double x,y;} points[limn];
int n;
double aria(0);

int main()
{
    fin>>n;
    for (int i=0; i<n; i++)
        fin>>points[i].x>>points[i].y;
    points[n] = points[0];

    for (int i=0; i<n; i++)
        aria += points[i].x*points[i+1].y - points[i].y*points[i+1].x;
    fout<<setprecision(5)<< 0.5 * aria;
    return 0;
}