#include <fstream>
#include <vector>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n, i;
vector < pair<float, float> > coord;
float a, x, y;
int main ()
{
    f >> n;
    for(i = 0; i < n; ++ i)
    {
        f >> x >> y;
        coord.push_back(make_pair(x, y));
    }
    for(i = 0; i < n; ++ i)
        a += coord[i].first * coord[(i+1 < n? i+1 : 0)].second - coord[(i+1 < n? i+1 : 0)].first * coord[i].second;
    a /= 2;
    g << a;
    return 0;
}