#include <bits/stdc++.h>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
int n;

struct punct {
    ///cine stie cunoaste
    long double x, y;
}v[100010];

signed main()
{
    fin >> n;
    for(int i=1; i<=n; i++) fin >> v[i].x >> v[i].y;
    v[n + 1] = v[1];
    long double rez = 0;
    for(int i=1; i<=n; i++) rez += (v[i].x * v[i + 1].y - v[i + 1].x * v[i].y);
    rez = 1.0 * abs(rez) / 2;
    fout << fixed << setprecision(5) << rez;

    return 0;
}