#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
struct punct
{
    double x, y;
};

inline double det(punct A, punct B)
{
    return A.x * B.y - A.y * B.x;
}

int main()
{
    int n;
    double s = 0;
    f >> n;
    punct a, b, c;
    f >> a.x >> a.y;
    c = a;
    for(int i = 0; i < n; i++)
    {
        f >> b.x >> b.y;
        s += det(a, b);
        a = b;
    }
    s += det(b, c);
    if(s < 0)s = -s;
    s /= 2;
    g << fixed << setprecision(6) << s;
    return 0;
}