#include<fstream>
#include<iostream>
#include<queue>
#include<vector>
#include<iomanip>
#define DN 100005
#define pb push_back
#define x first
#define y second
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n;
pair<int,int>a[DN];
double r;
double det(double l1,double c1,double l2,double c2)
{
    return l1*c2-c1*l2;
}
int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
        fin>>a[i].x>>a[i].y;
    n++;
    a[n]=a[1];
    for(int i=1;i<n;i++)
        r+=det(a[i].x,a[i].y,a[i+1].x,a[i+1].y)/2;
    fout<<fixed<<setprecision(5)<<r;
}