#include <iostream>
#include <fstream>
#include <cmath>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
#define MAX 120010
#define x first
#define y second
pair<int, int> a[MAX];


int main()
{
    int n, i;
    fin >> n;
    for(i = 1 ; i <= n ; i++)
    {
        fin >> a[i].x >> a[i].y;
    }
    long long arie = 0;
    a[n + 1] = a[1];
    for(i = 1 ; i <= n ; i++)
    {
        arie += 1LL * (a[i].x + a[i + 1].x)*(a[i].y - a[i + 1].y);
    }
    fout << abs(arie)/2.0 << '\n';
}