#include <bits/stdc++.h>
#define all(cont) cont.begin(), cont.end()
#define pb push_back
#define fi first
#define se second

using namespace std;

typedef pair <int, int> pii;
typedef vector <int> vi;
typedef long long ll;
typedef unsigned long long ull;

ifstream f ("aria.in");
ofstream g ("aria.out");

int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
#ifdef LOCAL_DEFINE
    freopen (".in", "r", stdin);
#endif
    g.precision (10);
    g << fixed;
    int n;
    f >> n;
    vector < pair <double, double> > v (n + 1);
    for (int i = 0; i < n; ++i) {
        f >> v[i].fi >> v[i].se;
    }
    v[n] = v[0];
    double ans = 0;
    for (int i = 0; i < n; ++i) {
        ans += (v[i].fi * v[i + 1].se - v[i + 1].fi * v[i].se);
    }

    g << abs (ans) / 2.0 << '\n';

    f.close();
    g.close();
    return 0;
}