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

using namespace std;

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

int n, mini_x=-2000000, mini_y=-2000000;
long double arie;

struct p{
    long double x, y;
}puncte[100005], pct_maxim;

double calculcare_arie(p punct1, p punct2, p punct3)
{
    return (punct1.x*punct2.y+punct2.x*punct3.y+punct3.x*punct1.y-punct3.x*punct2.y-punct2.x*punct1.y-punct1.x*punct3.y)/2;
}



int main() {
    freopen("aria.out","w",stdout);
    f >> n;
    for (int i=1; i<=n; i++)
    {
        f >> puncte[i].x >> puncte[i].y;
    }
    puncte[n+1]=puncte[1];
    pct_maxim.x=-1000005;
    pct_maxim.y=-1000005;
    for (int i=1; i<=n; i++)
    {
        arie+=((puncte[i].y+puncte[i+1].y)/2*(puncte[i].x-puncte[i+1].x));
    }
    if (arie<0)
        arie=-arie;
    g << setprecision(6) << fixed << arie;
    return 0;
}