#include <fstream>
#include <iomanip>
#define MAX 500001
#define FIN "aria.in"
#define FOUT "aria.out"

using namespace std;

double X[ MAX ], Y[ MAX ];
double sol = 0;
int N;

void read();
void solve();

int main() {

     read();
     solve();

   return (0);
};

void read() {

     int i;

     ifstream fin( FIN );

     fin>>N;

     for(i = 0; i < N; i++) fin>>X[i]>>Y[i];
             
     X[ N ] = X[ 0 ];
     Y[ N ] = Y[ 0 ]; 
};

void solve() {

     ofstream fout( FOUT );

     for(int i = 0; i < N; i++) sol += (X[ i ] * Y[ i + 1] - X[i + 1] * Y[ i ]);

     sol /= 2; if(sol<0) sol *= -1;
  
     fout<<fixed<<setprecision(8)<<sol;
}