#include <bits/stdc++.h>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
const  int ll = 100001;
struct punct
{
    long double x, y;
};
punct v[ll];
int n,i;
long double s = 0;
int main()
{

    fin>>n;
    for(i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    v[0].x=v[n].x,v[0].y=v[n].y;
    for(i=1;i<=n;i++)
        s+=(v[i-1].x*v[i].y-v[i].x*v[i-1].y);
    fout << fixed << setprecision(5) << s/2;
    return 0;
}