#include <fstream>
  #include <iostream>
  #include <vector>
  #include <iomanip>
  #include <algorithm>
using namespace std;

ifstream fin("aria.in");
ofstream fout("aria.out");
int n;

struct punct{
   long double x,y;
    
};

punct v[100005];
 

int determinant(punct a,punct b)
{
    long double u=(a.x*b.y)-(b.x*a.y);
    return u;
    
}
void aria()
{   int a=0;
    for(int i=0;i<n-1;i++)
    {
        a+=(determinant(v[i], v[i+1])/2);
     
    }
    a=a+(determinant(v[n-1], v[0])/2);
   
    fout<<setprecision(5)<<fixed;
    fout<<a;
}
int main( ) {
    fin>>n;
    
    
    for(int i=0;i<n;i++)
    {
        fin>>v[i].x>>v[i].y;
    }
    aria();
    return 0;
}