#include <fstream>
#include <utility>
#include <algorithm>
#define NMAX 100005
#define x first
#define y second
#define mp make_pair

using namespace std;

typedef pair<double,double> Point;
FILE* f=freopen("aria.in","r",stdin);
FILE* o=freopen("aria.out","w",stdout);

int n;
Point pts[NMAX];

double Surface()
{
    double s=0;
    for(int i=0;i<n;++i)
    {
        s+=(pts[i].x*pts[i+1].y-pts[i+1].x*pts[i].y)/2.0;
    }
    return s;
}

int main()
{
    scanf("%d",&n);
    for(int i=0;i<n;++i)
    {
        scanf("%lf%lf",&pts[i].x,&pts[i].y);
    }
    pts[n]=pts[0];

    printf("%.7lf",Surface());

    return 0;
}