#include<fstream>
#include<cstdio>
#include<iomanip>
#include<iostream>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
long double s,x[100003],y[100003];
int main()
{
    int n,i;
    fin>>n;
    for(i=1;i<=n;i++)
        fin>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];
    for(i=1;i<=n;i++)
        s=s+(x[i]*y[i+1]-x[i+1]*y[i])/2;
   fout << fixed << setprecision (5) <<s;
}