#include <fstream>

using namespace std;

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

struct punct
{
    double x, y;
}v[100001];

int main()
{
    int n;
    double s=0;
    f >> n;
    for(int i=1;i<=n;i++)
        f >> v[i].x >> v[i].y;

    for(int i=1;i<=n;i++)
        s += v[i].x * v[i+1].y - v[i+1].x * v[i].y;

    s += v[n].x * v[1].y - v[1].x * v[n].y;

    g <<(double) s/2;
    return 0;
}