#include <iostream>
#include <cstdio>
#include <cmath>

using namespace std;

int n;

struct coord
{
    double x,y;

}coordonate[100000];

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

void prelucrare()
{
    long double arie=0;
    for(int i=0; i<n-1; i++)
    {
        arie+=((coordonate[i].y+coordonate[i+1].y)*(coordonate[i+1].x-coordonate[i].x));
    }
    arie=abs(arie/2);

    printf("%Lf",arie);
}

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);

    citire();
    prelucrare();

    return 0;
}