#include <fstream>
#include <cmath>

using namespace std;

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

int n;
double sum;

struct punct{
    double x, y;
}pct[100005];

double arieTrapez(punct A, punct B)
{
    return (B.x-A.x)*(B.y+A.y);
}

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

    for(int i=1; i<=n; i++)
    {
        sum+=arieTrapez(pct[i], pct[i+1]);
    }
    sum/=2;
    g<<abs(sum);


    return 0;
}