#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

int main()
{
    int n,i;
    fin>>n;
    long long x1,y1;
    fin>>x1>>y1;
    long long x0=x1,y0=y1;
    long long arie=0;
    for (i=2; i<=n; i++)
    {
        long long x2,y2;
        fin>>x2>>y2;
        arie=arie+(x1*y2-x2*y1)/2.0;
        if(i==n)
            arie=arie+(x2*y0-x0*y2)/2.0;
        x1=x2;
        y1=y2;
    }
    fout<<setprecision(10)<<arie;
    return 0;
}