#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int main()
{
    int n;
    long double x0, y0, x, y, xN, yN, aria;
    fin>>n;
    fin>>x0>>y0;
    x = x0;
    y = y0;
    aria = 0;
    while(fin>>xN>>yN) {
        aria +=(x*yN-y*xN);
        x = xN;
        y = yN;
    }
    aria +=(xN*y0-yN*x0);
    if(aria < 0) fout<<'-';
    aria = aria / 2.0;
    fout<<setprecision(200)<<aria<<'\n';
    fin.close();
    fout.close();
    return 0;
}