#include <fstream>
#include <iomanip>
#include <utility>
#include <cstdlib>
#include <cmath>
#define x first
#define y second

using namespace std;
using db=long double;
using pdb=pair<db,db>;

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

const int N=1e5+2;

int n,i;
pdb v[N];

db area(){
    db a=0;
    for(i=1; i<=n; i++)
        a+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    return a/2.0;
}

int main()
{
    fin>>n;
    for(i=1; i<=n; i++)
        fin>>v[i].x>>v[i].y;
    v[n+1]=v[1];

    fout<<fixed<<setprecision(5)<<abs(area());
    return 0;
}