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

struct Point {int x, y;};

inline int det(Point p1, Point p2, Point p3)
{
    return p1.x*p2.y - p3.x*p2.y + p2.x*p3.y - p1.x*p3.y + p3.x*p1.y - p1.y*p2.x;
}

int main()
{
    fstream f("aria.in", ios::in);
    int n, i;
    long double s;
    vector<Point> p;

    f >> n;
    p.resize(n);

    for(i = 0; i < n; ++i)
        f >> p[i].x >> p[i].y;
    f.close();

    s = 0;
    for(i = 1; i < n-1; ++i)
        s += abs(det(p[0],p[i],p[i+1])) / 2.0L;

    f.open("aria.out", ios::out);
    f << s;

    return 0;
}