#include <stdio.h>

#include <cmath>



using namespace std;



struct punct

{

    double x, y;

}a[100010];



int main()

{

    int n;

    double arie=0;

    FILE *f=fopen("aria.in", "r"), *g=fopen("aria.out", "w");

    fscanf(f, "%d", &n);

    for (int i=1; i<=n; i++)

        fscanf(f, "%lf%lf", &a[i].x, &a[i].y);

    for (int i=1; i<n; i++)

        arie=arie+(a[i].x*a[i+1].y-a[i].y*a[i+1].x);

    arie=arie+(a[n].x*a[1].y-a[n].y*a[1].x);

    arie=abs(arie/2);

    fprintf(g, "%.5lf", arie);

    return 0;

}