#include<stdio.h>
struct aria
{
    double x,y;
};
aria v[100005];
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int n,i;
    double s=0;
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%lf%lf",&v[i].x,&v[i].y);
    v[0]=v[n];
    for(i=0;i<n;i++)
        s=s+v[i].x*v[i+1].y-v[i].y*v[i+1].x;
    printf("%.5lf\n",0.5*s);
return 0;
}