#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct Punct{
double x,y;
};
int main()
{
    int n;
    double  s = 0;
    f>>n;
    Punct P[n+1];
    for(int i = 1 ; i<= n ; i++)
        f>>P[i].x>>P[i].y;
    P[++n] = P[1];
    for(int i = 1 ; i < n ; i++)
        s += (P[i].x*P[i+1].y - P[i+1].x*P[i].y);
    g<<setprecision(5)<<fixed<<s/2.0;
    return 0;
}