#include <fstream>
using namespace std;

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

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

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