#include <bits/stdc++.h>

using namespace std;

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

struct coord
{
    long double x, y;
}v[100001];
long double sol;

int main()
{
    ios::sync_with_stdio(false);
    int n;
    fin>>n;
    for ( int i = 0; i < n; ++i )
        fin>>v[i].x>>v[i].y;
    v[n] = v[0];
    for ( int i = 0; i < n; ++i )
        sol += v[i].x*v[i+1].y - v[i].y*v[i+1].x;
    fout<<setprecision(6)<<fixed<<abs(sol)/2;
}