#include <bits/stdc++.h>
#include <iomanip>
using namespace std;
ifstream fin ("aria.in");
ofstream fout("aria.out");
struct point
{
    long double x,y;
}v[100010];
long double area;
int n,i,j;
long double makeArie(point a,point b)
{
    return a.x*b.y-a.y*b.x;
}
int main()
{
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    v[n+1]=v[1];
    for(i=1;i<=n;i++)
        area=area+makeArie(v[i],v[i+1])/2.0;
    fout<<fixed<<setprecision(5)<<area;
    return 0;
}