#include <stdio.h>
#include <vector>
#include <utility>
#include <math.h>
using namespace std;
std::vector< std::pair<double,double> > a;
int n;
double x,y,sol;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for(int i=1;i<=n;++i)
    {
        scanf("%lf %lf",&x,&y);
        a.push_back( std::make_pair(x,y) );
    }
    a.push_back(a[0]);
    for(int i=0;i<a.size()-1;++i)
    {
        sol = sol + (a[i].first*a[i+1].second-a[i+1].first*a[i].second);
    }
    printf("%lf",fabs(sol/2));
    return 0;
}