#include <iostream>
#include <fstream>
#include <vector>
#include <iomanip>
#include <cmath>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
struct poz{
    int x,y;
};
vector<poz>v;
int main()
{
    int x,y,n;
    in>>n;
    for(int i=1;i<=n;i++){
        in>>x>>y;
        v.push_back((poz){x,y});
    }
    double arie=0;
    for(int i=1;i<n;i++)
        arie+=(double)((v[i-1].x-v[i].x)*(v[i-1].y+v[i].y));
    out<<setprecision(5)<<arie*0.5;
    return 0;
}