#include <bits/stdc++.h>

using namespace std;

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

int n;

struct Puncte
{
    double x,y;

}a[100005];

int main()
{
    fin>>n;

    for(int i=1;i<=n;i++)
        fin>>a[i].x>>a[i].y;

    double sol=0;

    a[n+1].x=a[1].x;
    a[n+1].y=a[1].y;

    for(int i=1;i<=n;i++)
        sol+=(a[i].x*a[i+1].y-a[i].y*a[i+1].x);

    fout<<fixed<<setprecision(5)<<(sol/2.0)<<"\n";

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