#include <bits/stdc++.h>

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

struct db{
    long double x, y;
}a[100005];

int n;

void Citire()
{
    fin >> n;
    for(int i = 1; i <= n; i++)
        fin >> a[i].x >> a[i].y;

    a[n + 1] = a[1];
    n++;
}

void Rez()
{
    long double arie = 0.0;

    for(int i = 2; i <= n; i++)
        arie += a[i - 1].x * a[i].y - a[i - 1].y * a[i].x;

    fout << fixed << setprecision(7) << arie / 2.0 << "\n";
}
int main()
{
    Citire();
    Rez();
    return 0;
}