#include <fstream>

using namespace std;

ifstream fin ("aria.in");
ofstream fout ("aria.out");

struct pct{
double x, y;
};

int n;
double A;
pct p[100005];

double det( pct a, pct b, pct c);

int main()
{
    int i;
    fin>>n;
    for(i=1; i<=n; ++i)
        fin>>p[i].x>>p[i].y;
    for(i=3; i<=n; ++i)
        A=A+det(p[1], p[i-1], p[i]);

    fout<<A<<'\n';
    return 0;
}

double det( pct a, pct b, pct c)
{
    return ((b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x))/2;
}