# include <cstdio>
# include <cmath>
using namespace std;
int n,i;
double sol;
struct punct
{
    double x,y;
}a[100005];
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for(i=0;i<n;++i)
        scanf("%lf%lf",&a[i].x,&a[i].y);
    a[n]=a[0];
    for(i=0;i<n;++i)
        sol+=a[i].x*a[i+1].y-a[i+1].x*a[i].y;
    printf("%lf\n",fabs(sol/2.0));
    return 0;
}