#include <fstream>
#include <iomanip>
#define NMAX 1000001
using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

struct punct
{   float x, y;
} a[NMAX+1];

int i, j, n;
float A;

int main()
{   f>>n;
    for (i=1; i<=n; ++i)
        f>>a[i].x>>a[i].y;
    a[n+1]=a[1];
    for (i=1; i<=n; ++i)
        A+=(a[i].x*a[i+1].y-a[i+1].x*a[i].y)/2;
    g<<setprecision(5)<<fixed;
    g<<A<<'\n';
    return 0;
}