#include <fstream>
#include <algorithm>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct ar{
double x,y;
}a[100005];
int n;
double aria()
{
    double s = 0;
    for ( long long i = 1 ; i <= n ; i++ )
        s += (a[i].x*a[i+1].y-a[i+1].x*a[i].y);
    s /= 2;
     return s;
}

int main()
{
    f >> n;
    for ( long long i = 1 ; i <= n ; i++ )
        f >> a[i].x >> a[i].y;
    a[n+1] = a[1];
     g << fixed << setprecision(5) << aria();
    return 0;
}