#include <fstream>
#include <iomanip>
#define dim 100010
using namespace std;
struct punct {
    long double first, second;
} v[dim];;
long double sol;
int n, i;
ifstream fin ("aria.in");
ofstream fout ("aria.out");

long double aria (punct a, punct b, punct c)
{
    return(b.first-a.first)*(c.second-a.second)-(c.first-a.first)*(b.second-a.second);
}

int main () {
    fin>>n;
    for (i=1;i<=n;i++) {
        fin>>v[i].first>>v[i].second;
    }
    v[0]=v[n];
    for (i=0;i<n;i++) {
        sol+=aria(v[0], v[i], v[i+1]);
    }
    fout<<setprecision(5)<<fixed<<sol/2.0;

    return 0;
}