#include <fstream>
#include <stdlib.h>
#include <iomanip>

using namespace std;

ifstream cin("aria.in");
ofstream cout("aria.out");

int n; double a,b,a1,b1,a2,b2;
double aria;

int main()
{
    cin>>n>>a>>b>>a1>>b1;
    for (int i=2;i<=n;i++)
    {
        cin>>a2>>b2;
        aria+=(a1-a)*(b2-b)-(a2-a)*(b1-b);
        a1=a2;b1=b2;
    }

    cout<<fixed;
    cout<<setprecision(5)<<abs(aria/2)<<'\n';
    cin.close();cout.close();
    return 0;
}