#include <fstream>
#include<iomanip>
#define ab(a) ((a < 0) ? -a : a)

#define NMAX 100005
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

double x[NMAX],y[NMAX];
int main()
{
    int N;
    fin>>N;
    for(int i = 1; i<=N; i++)
        fin>>x[i]>>y[i];

    x[0] = x[N], y[0] = y[N];
    double aria = 0;
    for(int i = 0; i<=N;i++)
    {
        aria += (x[i]*y[i+1]-x[i+1]*y[i]);
    }
    aria/=2.0;
    fout<<fixed;
    fout<<setprecision(5)<<(double)ab(aria)<<'\n';
    return 0;

}