#include <fstream>
#include <cmath>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct point
{
    int x, y;
};
double area(point a, point b, point c)
{
    a.x-=c.x, a.y-=c.y;
    b.x-=c.x, b.y-=c.y;
    return fabs((double)(a.x*b.y-a.y*b.x)/2);
}
long double arie;
int n, i, j;
int main()
{
    fin>>n;
    point a;
    fin>>a.x>>a.y;
    point b;
    fin>>b.x>>b.y;
    point c;
    for(i=3;i<=n;i++)
    {
        fin>>c.x>>c.y;
        arie+=area(a,b,c);
        b.x=c.x,b.y=c.y;
    }
    fout<<arie;
    return 0;
}