#include <fstream>
using namespace std;

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

int n;
pair < double, double > V[1000010];

int main()
{
    fin >> n;
    for (int i = 1; i <=; n ++) fin >> V[i].first >> V[i].second;
    V[n + 1] = V[1];
    for (int i = 1; i <= n; i ++) V[0] += (V[i].first * V[i + 1].second - V[i].second * V[i + 1].first);
    fout << V[0] / 2 << '\n';
    return 0;
}