#include <fstream>
#include <vector>
#include <algorithm>

using namespace std;

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

int n;
vector <pair<int, int>> v;

int main() {
    fin >> n;
    for(int i = 0 ; i < n ; ++ i) {
        int x, y;
        fin >> x >> y;
        v.push_back(make_pair(x, y));
    }
    int ans = 0;
    v.push_back(v[0]);
    for(int i = 0 ; i < n ; ++ i)
        ans = ans + v[i].first * v[i + 1].second - v[i + 1].first * v[i].second;
    fout << 0.5 * ans << '\n';
}