#include <fstream>
#include <iomanip>
using namespace std;
ifstream f ("aria.in");
ofstream g ("aria.out");
struct punct
{
    double x, y;
}a[100010];

int n, i, j;
double p, s;
int main (){

    f >> n;
    for (i=1; i<=n; i++)
        f >> a[i].x >> a[i].y;

//    for (i=1; i<=n; i++) printf("%f %f\n", a[i].x, a[i].y);
//    printf("\n");

    a[n+1]=a[1];
    for (i=1; i<=n; i++){
        p=(a[i].x*a[i+1].y-a[i+1].x*a[i].y)/2;
        s+=p;
    }
    g <<fixed << setprecision(5) << s;
    return 0;
}