#include<cstdio>
#include<fstream>
#include<cmath>
using namespace std;
int n;
double sol=0.0;
pair <double,double> v[100001];
int main()
{
    freopen("aria.out","w",stdout);
    freopen("aria.in","r",stdin);
    scanf("%d",&n);
    for(int i=1;i<=n;i++)
        scanf("%lf %lf",&v[i].first,&v[i].second);
    v[n+1]=v[1];
    for(int i=1;i<=n;i++)
        sol+=(v[i].first*v[i+1].second-v[i].second*v[i+1].first);
    printf("%.6lf",fabs(sol/2.00));
    return 0;
}