#include <fstream>
#include <cmath>

using namespace std;


FILE * fout;
FILE * fin;

int n, i;
double aria;
struct v{double x, y;} V[100005];

int main()
{
    fin = fopen("aria.in", "r");
    fout = fopen ("aria.out","w");
    fscanf(fin, "%d", &n);
    for (i=0; i<n; i++)
    {
        fscanf(fin, "%lf %lf", &V[i].x, &V[i].y);
    }
    V[n].x=V[0].x;
    V[n].y=V[0].y;
    for (i=0; i<n; i++)
    {
        aria += V[i].x*V[i+1].y-V[i].y*V[i+1].x;
    }
    fprintf(fout, "%f", abs(aria)/2.0);
    return 0;
}