#include <iostream>
#include <fstream>

using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct pct{double x,y;} v[100005];
double A=0;
int n,i,j;

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