#include <bits/stdc++.h>

using namespace std;

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

double getArea(pair<double, double> a, pair<double, double> b)
{
    return (a.first-b.first)*(a.second+b.second)/2;
}

int main()
{
    int n;
    fin>>n;
    vector<pair<double, double  >> v(n);
    for(int i=0; i<n; i++)
    {
        fin>>v[i].first>>v[i].second;
    }
    double area=0;
    for(int i=0; i<n-1; i++)
    {
        area+=getArea(v[i], v[i+1]);
    }
    area+=getArea(v[n-1], v[0]);
    fout<<area;
    return 0;
}