#include <bits/stdc++.h>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");
int n, ind1, ind2;
long double arie;
struct punct{
    long double x, y;
}puncte[100005], fixat;

long double determinant(punct a, punct b, punct c)
{
    return (a.x-c.x)*(b.y-c.y)-(a.y-c.y)*(b.x-c.x);
}

int main()
{
    f >> n;
    for (int i=1; i<=n; ++i)
    {
        f >> puncte[i].x >> puncte[i].y;
    }
    fixat.x=0, fixat.y=0;
    ind2=1;
    for (int i=2; i<=n; ++i)
    {
        ind1=ind2;
        ind2=i;
        arie+=determinant(fixat,puncte[ind1],puncte[ind2]);
    }
    arie+=determinant(fixat,puncte[n],puncte[1]);
    g << arie/2;
    return 0;
}