#include <cstdio>
#include <cmath>
using namespace std;
struct PCT
{
    double x,y;
}v[100005];
int n;
double ans;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int i;
    scanf("%d",&n);
    for(i=1;i<=n;i++)
        scanf("%lf%lf",&v[i].x,&v[i].y);
    for(i=1;i<n;i++)
            ans+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    ans+=v[n].x*v[1].y-v[1].x*v[n].y;
    ans=fabs(ans/2.0);
    printf("%.5f\n",ans);
    return 0;
}