#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

struct a{
    int x, y;
}a[100001];

long double s;
int i, n;

int main()
{
    f >> n;
    for (i = 1; i <= n; i++)
    {
        f >> a[i].x >> a[i].y;
        if (i >= 2)
            s += (a[i-1].x*a[i].y - a[i].x*a[i-1].y);
    }
    s = fabs(s + a[n].x*a[1].y - a[1].x*a[n].y)/2;
    g << s;
    return 0;
}