#include <fstream>
#include <cmath>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");

struct point {double x, y;} v[100005];
int n,i;

double area_3(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-b.x*a.y)/2);
}

double area_n(int n,point p[]){
    double x=0;
    for (int i=2; i<n; i++)
        x+=area_3(p[1],p[i],p[i+1]);
    return x;
}

int main()
{
    f>>n;
    for (i=1; i<=n; i++)
        f>>v[i].x>>v[i].y;
    double x=area_n(n,v);
    g<<x;
    return 0;
}