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

using namespace std;

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

#define nmax 100003
struct p{double x,y ;};
p v[nmax];
long double aria;

int main()
{
    int n,i;
    fin>>n;

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

    for(i=1; i<=n; i++)
        aria+=(v[i].x*v[i+1].y - v[i].y*v[i+1].x);
    fout<<setprecision(5)<<aria/2.0;


    fin.close();
    fout.close();
    return 0;
}