#include <iostream>

using namespace std;

struct Point

{

    int x, y;

};

Point a[];

int main()

{

    int n;

    while (cin >> n && n != )

    {

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

        {

            cin >> a[i].x >> a[i].y;

        }

        double area = 0.0;

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

        {

            int x1 = a[i].x;

            int y1 = a[i].y;

            int x2 = a[i + ].x;

            int y2 = a[i + ].y;

            area += 0.5*abs(x1*y2 - y1*x2);

        }

        int x1 = a[].x;

        int y1 = a[].y;

        int x2 = a[n].x;

        int y2 = a[n].y;

        area += 0.5*abs(x1*y2 - y1*x2);

        printf("%.1lf\n", area);

    }

}