#include<cstdio>
#include<algorithm>
#define Nmax 100009
#define x first
#define y second
 
using namespace std;
 
int n,i;
double s;
pair<double,double> a[Nmax];
 
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];
 
    for (i=2;i<=n+1;i++)
        s+=(a[i-1].x*a[i].y - a[i-1].y*a[i].x);
    if (s<0) s=-s;
 
    printf("%lf",s/2.0);
}