#include <iostream>
#include <fstream>
#include <cmath>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");

const int nmax = 100005;

struct point{
double x;
double y;
};

point vec[nmax];

int main()
{
    int n;
    fin >>n;

    for(int i=0;i<n;i++){
        fin >> vec[i].x;
        fin >> vec[i].y;
    }
    vec[n] = vec[0];

    double aria=0;

    for(int i=0;i<n;i++){
        aria+=(vec[i].x*vec[i+1].y - vec[i+1].x*vec[i].y);
    }

    fout << fabs(aria/2.0);

    return 0;
}