#include <fstream>
#include <cstdio>
#define NMAX 100005

using namespace std;

double x[NMAX],y[NMAX];
double s;
int n,i;

int main()
{
    ifstream f("aria.in");
     freopen("aria.out","w",stdout);
    f>>n;
    for(i=1;i<=n;++i) f>>x[i]>>y[i];
    x[0]=x[n];y[0]=y[n];
    double s=0;
    for(i=0;i<n;++i)
    {
        s = s + (x[i]*y[i+1] - x[i+1]*y[i]);
    }
    s=s/2;
    printf("%0.5lf",s);
    return 0;
}