#include <bits/stdc++.h>
using namespace std;

string const& task("aria");
ifstream fin(task + ".in");
ofstream fout(task + ".out");

int const N(1e5 + 5);

struct Point {
    long double x, y;
};

Point P[N];
int n;
long double arie;

signed main() {

    fin >> n;
    for (int i = 1; i <= n; ++i)
        fin >> P[i].x >> P[i].y;
    for (int i = 1; i <= n; ++i) {
        int j = i + 1;
        if (j == n + 1)
            j = 1;
        arie += (P[j].x - P[i].x) * (P[j].y + P[i].y);
    }
    arie = abs(arie) / 2;
    fout << fixed << setprecision(10) << arie;

    return 0;
}