#include <bits/stdc++.h>
#define DIM 100001

using namespace std;

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

pair <long double, long double> v[DIM];

int n, i;

long double sol;

long double area(pair <long double, long double> a, pair <long double, long double> b, pair <long double, long double> c){
    return (c.first - a.first) * (b.second - a.second) - (b.first - a.first) * (c.second - a.second);
}

signed main(){

    ios :: sync_with_stdio(false);
    fin.tie(0);
    fout.tie(0);

    fin >> n;

    for(i=1;i<=n;i++)
        fin >> v[i].first >> v[i].second;

    v[0] = v[n];

    for(i=0;i<n;i++)
        sol += area(v[0], v[i], v[i + 1]);

    long double answer = -sol;

    answer /= 2;

    fout << setprecision(6) << fixed << answer << "\n";

    fin.close();
    fout.close();
    return 0;

}