#include <iostream>
#include <fstream>

#define MAX 100002
using namespace std;
int n;
struct element
{
    double first;
    double second;
}v[MAX];

void read()
{
    scanf("%d ",&n);
    double x,y;
    for(int i=0;i<n;i++)
        {
            scanf("%lf %lf",&x,&y);
            v[i].first=x;
            v[i].second=y;
        }
    v[n]=v[0];
}

double arie(element x,element y)
{
    return (x.first*y.second-x.second*y.first);
}

double solve()
{
    double sum=0;
    for(int i=0;i<n;i++)
        sum+=arie(v[i],v[i+1]);
    return sum/2;
}
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    read();
    printf("%.5lf ",solve());
    return 0;
}