#include <cstdio>
using namespace std;
struct punct
{
    double x,y;
}a[100002];
int n,i;
double aria;
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]=a[1];
    aria=0;
    for(i=1;i<=n;i++)
        aria+=(a[i].x*a[i+1].y-a[i].y*a[i+1].x);
    aria/=2.0;
    if(aria<0)
        aria=-aria;
    printf("%lf\n",aria);
    return 0;
}