#include <iostream>
#include <math.h>
#include <fstream>
#include <iomanip>
#include <stdio.h>
#include <stdlib.h>
using namespace std;
ifstream fin("aria.in");
ofstream g("aria.out");
struct puncte{double x , y;}v[100010];;
long long int i,j,n;
double a,b,c,p,s;
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++)
    {
        s+=v[i].x*v[i+1].y-v[i+1].x*v[i].y;
    }
    if(s<0) s=-1*s;
    g<<fixed<<setprecision(5)<<s/2;;
}