using namespace std;
#include <fstream>
ifstream fin("aria.in");
FILE *fout = fopen("aria.out", "w");

const int vfMax = 100000;

struct _punct {double x, y;};
struct _poligon
{
    int vf;
    _punct v[vfMax + 2];
};

_poligon p;

int main()
{
    int i;
    double s;
    fin >> p.vf;
    for(i = 1; i <= p.vf; ++i) fin >> p.v[i].x >> p.v[i].y;
    p.v[0] = p.v[p.vf]; p.v[p.vf + 1] = p.v[1];
    for(s = 0, i = 1; i <= p.vf; ++i)
        s += p.v[i].x * (p.v[i-1].y - p.v[i+1].y);
    if(s < 0) s = -s; s /= 2;
    fprintf(fout, "%.5lf", s);
    return 0;
}