#include <iostream>
#include <bits/stdc++.h>
#define VMAX 100005
#define INF 2147000000
#define double long double
using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");

struct pct{
double x,y;
};
pct puncte[VMAX];

double arie(pct a, pct b)
{
    return a.x*b.y-b.x*a.y;
}

signed main()
{
    long long int n,m,i,j,k,t,q,nr,p;
    double suma;
    fin>>n;
    for(i=0;i<n;i++)
        fin>>puncte[i].x>>puncte[i].y;

    puncte[n]=puncte[0];
    suma=0;

    for(i=1;i<=n;i++)
        suma+=arie(puncte[i-1],puncte[i]);

    suma=abs(suma)/2;
    fout<<fixed<<setprecision(6)<<suma<<'\n';

    return 0;
}