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

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

const int maxN = 100002;
struct punct {
    double x, y;
};
punct v[maxN];

int main()
{
    int n;
    double s = 0;
    in >> n;
    for(int i = 1; i <= n; i++)
        in >> v[i].x >> v[i].y;
    v[n+1].x = v[1].x, v[n+1].y = v[1].y;
    for(int i = 1; i <= n; i++)
        s += (v[i].x * v[i+1].y - v[i+1].x * v[i].y);
    out << fabs(s * 0.5);
    return 0;
}