#include <bits/stdc++.h>

using namespace std;

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


struct Punct
{
    double x, y;
};

Punct a[100005];
int n;


int main()
{
    double aria = 0;
    fin >> n;
    for (int i = 1; i <= n; i++)
        fin >> a[i].x >> a[i].y;
    a[n + 1] = a[1];
    for (int i = 1; i <= n; i++)
        aria += (a[i].x * a[i + 1].y - a[i].y * a[i + 1].x);
    aria /= 2;
    if (aria < 0)
        aria = -aria;
    fout << setprecision (5) << fixed << aria << "\n";
    return 0;
}