#include<cstdio>
int i,n;
double s;
struct andreea
{
    double x,y;
}a[100004];
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    for(i=1;i<=n;i++)
    {
        scanf("%lf%lf",&a[i].x,&a[i].y);
    }
    a[n+1].x=a[1].x;
    a[n+1].y=a[1].y;
    for(i=1;i<=n;++i)
        s=s+a[i].x*a[i+1].y-a[i+1].x*a[i].y;
    printf("%lf",s/2.0);
    return 0;
}