#include <fstream>
#include <iomanip>

using namespace std;

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

double x[100010],y[100010];

int main (){
    int n,i;
    double arie=0;
    in>>n;
    for(i=1;i<=n;i++) in>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(i=1;i<=n;i++) arie=arie+(x[i]*y[i+1]-x[i+1]*y[i]);
    out<<fixed<<setprecision(5)<<arie/2;
    return 0;
}