#include <fstream>
#include <cmath>
#include <iomanip>

using namespace std;

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

long long n,i,x[100010],y[100010],xstart,ystart;
long double aria;

int main()
{
    cin>>n;
    for(i=1;i<=n;i++)
        cin>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(i=1;i<=n;i++)
        aria+=x[i]*y[i+1]-y[i]*x[i+1];
    cout <<fixed<<setprecision(5)<<aria/(2.0);
    return 0;
}