#include <iostream>
#include <cstdio>
#define cout cerr
#define MAX 100001
using namespace std;

int n;
pair <double, double> puncte[MAX];
double x,y;
double A;

double Arie(double xa,double ya, double xb,double yb)
{
    return xa*yb-ya*xb;
}

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);
        //cout<<x<<" "<<y<<endl;
        puncte[i]=make_pair(x,y);
    }


    for(int i=1; i<n; i++)
    {
        A+=Arie(puncte[i].first,puncte[i].second,puncte[i+1].first,puncte[i+1].second);

    }
    A+=Arie(puncte[n].first,puncte[n].second,puncte[1].first,puncte[1].second);

    if(A>0)
        printf("%.5lf",A/2);
    else  printf("%.5lf",-A/2);
    return 0;
}