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

using namespace std;

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

struct punct
{
    double x;
    double y;
} puncte[100001];

double sum;
int n;

int main()
{
    f >> n;
    for(int i = 0; i < n; i++)
        f >> puncte[i].x >> puncte[i].y;
    puncte[n].x = puncte[0].x;
    puncte[n].y = puncte[0].y;
    for(int i = 0; i < n; i++)
        sum += puncte[i].x * puncte[i + 1].y - puncte[i + 1].x * puncte[i].y;
    sum /= 2;
    g << setprecision(5) << abs(sum);
}