#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");
struct segment{
    int x,y;
};
segment v[100005];

int n;
long long s;
int main()
{
    fin>>n;
    for(int i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    for(int i=1;i<n;i++)
        {s+=((v[i].x*v[i+1].y)-(v[i+1].x*v[i].y));
        //cout<<s<<endl;
        }
        s+=((v[n].x*v[1].y)-(v[1].x*v[n].y));

    if(s<0)
    s*=(-1);
    fout<<setprecision(6)<<fixed<<double(s/2);

    return 0;
}