#include <fstream>
#include <iomanip>

using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");

struct point{
long long x, y;
}p[100001];
int n,i;

long double arie(point p1,point p2,point p3)
{
    return (long double)(p1.x*p2.y + p1.y*p3.x + p2.x*p3.y - p1.x*p3.y - p1.y*p2.x - p2.y*p3.x)/2;
}

int32_t main()
{
    cin>>n;
    for(i=1;i<=n;i++)
    {
        cin>>p[i].x>>p[i].y;
    }
    long double arietot = 0;

    for(i=1;i<n;i++)
    {

        arietot+=arie({0,0},p[i],p[i+1]);
    }
    arietot+=arie({0,0},p[n],p[1]);

    cout<<fixed<<setprecision(5)<<arietot;
    return 0;
}