#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int N;
double rez,x[100010],y[100010];
int main()
{
    int i,j;
    fin>>N;
    for(i=1;i<=N;i++)
    {
        fin>>x[i]>>y[i];
    }
    rez=0.5 * (x[N]*y[1]-x[1]*y[N]);
    for(i=1;i<N;i++)
    {
        rez=rez+0.5 * (x[i]*y[i+1]-x[i+1]*y[i]);
    }
    fout<<fixed<<setprecision(7)<<rez;
}