#include <cstdio>
#include <cmath>
using namespace std;
struct punct
{
    double x;
    double y;
}a[10001];
int n,i;
double suma;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d\n",&n);
    for (i=1; i<=n; i++) scanf("%lf %lf",&a[i].x,&a[i].y);
    suma=0;
    for (i=1; i<n; i++)
        suma+=(a[i].x*a[i+1].y-a[i+1].x*a[i].y);
    suma+=a[n].x*a[1].y-a[1].x*a[n].y;
    printf("%.2lf",fabs(suma/2));
    return 0;
}