#include <cstdio>

using namespace std;
int x[100004],y[100004],i,n;
double a,a1;
int main()
{
    freopen ("aria.in","r",stdin);
    freopen ("aria.out","w",stdout);
    scanf ("%d", &n);
    for (i=1;i<=n;i++)
        scanf ("%d %d", &x[i], &y[i]);
    x[n+1]=x[1];
    y[n+1]=y[1];
    a=0.0;
    for (i=1;i<=n;i++)
    {
        a1=(1.0*x[1]*y[2]-1.0*y[1]*x[2])/2.0;
        a=0.0+a+a1;
    }
    printf ("%f", 1.0*a);
    return 0;
}