#include <fstream>
using namespace std;

ifstream cin("aria.in");
ofstream cout("aria.out");

double arie;

struct punct{

    int x , y;
};

punct a , b;

int n;

int main()
{
    cin >> n;

    cin >> a.x >> a.y;

    punct og;

    og.x = a.x , og.y = a.y;

    for(int i = 2 ; i <= n ;i++){

        cin >> b.x >> b.y;

        arie += ((a.x*b.y)-(a.y*b.x));

        a = b;
    }

    arie += ((a.x*og.y)-(a.y*og.x));

    cout << arie/2.0;

    return 0;
}