#include <iostream>
#include <fstream>
#include <algorithm>
#include <iomanip>
#define MAX 100005

using namespace std;

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

int n;
double aria;

struct pct
{
    double x,y;

}v[MAX];

int main()
{
    f >> n;

    for(int i = 1; i <= n ; ++i)
        f >> v[i].x >> v[i].y;

    v[n+1] = v[1];

    for(int i = 1 ; i <= n ; ++i)
        aria += ( v[i].x * v[i+1].y - v[i+1].x * v[i].y);

    g << fixed << setprecision(5) << aria/2;
    return 0;
}