#include <fstream>
#include <iostream>
#include <bits/stdc++.h>

using namespace std;
static inline double arie( double x2, double x1, double y2, double y1 )
{
   return ( x2 - x1 ) / 2 * ( y2 + y1 );
}

double a[100000];
double b[100000];

int main() {

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

   int n, i;
   long double sum = 0;

   fin >> n;
   for ( i = 0; i < n; i ++ ) {
       fin >> a[i] >> b[i];
   }

   for ( i = 0; i < n - 1; i ++ )
   {
       sum += arie( a[i + 1], a[i], b[i + 1],  b[i] );
   }

   sum += arie( a[0], a[n - 1], b[0], b[n - 1] );
   sum = sum > 0 ? sum : -sum;
   fout << setprecision( 6 ) << fixed << sum;
   return 0;

}