#include <bits/stdc++.h>

using namespace std;
const int NMAX = 50005;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n, m;
int main()
{
    fin>>n;
    vector<pair<int, int>> points(n);
    for(int i = 0;i<n;i++)
        fin>>points[i].first>>points[i].second;
    double sum = 0;
    points.push_back(points[0]);
    for(int i = 0;i<n;i++)
        sum+=(points[i].first*points[i+1].second)-
                (points[i+1].first*points[i].second);
    fout<<abs((double)sum/2);
    return 0;
}