#include <iostream>
#include <fstream>
#include <iomanip>
#define INT long double
#define MAXI 100001
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
struct punct
{
    INT x;
    INT y;
    INT ind;
}v[MAXI];
INT ariedr(INT x1,INT y1,INT x2,INT y2)
{
    return x1*y2-x2*y1;
}
int main()
{
    int n,i;
    INT ar=0;
    fin>>n;
    for(i=1;i<=n;i++){
        fin>>v[i].x>>v[i].y;
    }
    for(i=1;i<=n-1;i++){
        ar+=ariedr(v[i].x,v[i].y,v[i+1].x,v[i+1].y);
    }
    ar+=ariedr(v[n].x,v[n].y,v[1].x,v[1].y);
    fout<<fixed<<setprecision(5)<<ar/2.0;
}