#include <bits/stdc++.h>
using namespace std;

struct Punct
{
    double x, y;
};
Punct a[100005];
int n;
ifstream fin("aria.in");
ofstream fout("aria.out");

int main()
{
    long 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 tr. A[i] A[i+1], 0
        aria += (a[i].x * a[i+1].y - a[i].y * a[i+1].x);
    aria /= 2;
    if (aria < 0) aria = -aria;
    fout << setprecision(6) << fixed << aria << "\n";
    fout.close();
    return 0;
}