#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct punct
{
    double x,y;
}v[999999];
int main()
{int n;
double arie=0;
f>>n;
for(int i=1;i<=n;i++)
    f>>v[i].x>>v[i].y;
v[n+1]=v[1];
for(int i=1;i<=n;i++)
{
    arie+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
}
    g<<fixed<<setprecision(5)<<arie/2;

    return 0;
}