#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

int main()
{
    int N;
    double x0,x1,x2,y0,y1,y2,s=0;
    f >> N;
    f >> x0 >> y0;
    x1=x0;
    y1=y0;
    s = 0;
    for (int i=2; i<=N; i++)
    {
        f >> x2 >> y2;
        s += x1 * y2 - x2 * y1;
        x1 = x2;
        y1 = y2;
    }
    s += x2*y0 - x0*y2;
    s /= 2;
    g <<  setprecision(5) << s;
    return 0;
}