#include <fstream>
#include <cmath>
#include <iomanip>
#define nmax 100005
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n;
double sol;
struct punct {double x;double y;};
punct p[nmax];


double det(const punct &a,const punct &b,const punct &c)
{return ((c.x-a.x)*(b.y-a.y)-(b.x-a.x)*(c.y-a.y))/2.0;
}


int main(){
    int i,j;
    f>>n;
    for (i=1;i<=n;i++) f>>p[i].x>>p[i].y;
    p[n+1]=p[1];
    for (i=1;i<=n;i++) sol+=det(p[0],p[i],p[i+1]);

    g<<setprecision(6)<<fixed<<abs(sol);
return 0;
}