#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

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

double sum;
int n;

struct poligon{
    double x,y;
}a[100002];

int main(){
    f>>n;
    for(int i=1; i<=n; ++i)
        f>>a[i].x>>a[i].y;
    if(n>2){
        a[0].x=a[n].x;
        a[0].y=a[n].y;
        for(int i=1; i<=n; ++i)
            sum=sum+(double)((a[i-1].x-a[i].x)*(a[i-1].y+a[i].y))/2;
        g<<fixed<<setprecision(6)<<sum<<'\n';
    }
    else g<<"0\n";
}