#include <iostream>
#include <fstream>
#include <cmath>
#define mp make_pair
#define MAXN 100001
#define INFILE "aria.in"
#define OUTFILE "aria.out"
#define x first
#define y second
using namespace std;
ifstream f(INFILE);
ofstream g(OUTFILE);
int n;
pair<double,double> v[MAXN];
long double res;
int main()
{
    f>>n;
    for(int i=1;i<=n;i++)
        f>>v[i].x>>v[i].y;
    res=(v[n].x*v[1].y-v[1].x*v[n].y)/2;
    for(int i=1;i<n;i++)
        res+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y)/2;
    g<<abs(res);
    f.close();
    g.close();
    return 0;
}