#include <cstdio>
#include <iomanip>
#include <iostream>

using namespace std;

const int nmax = 100005 ;

struct poz
{
    double x ;
    double y ;
} v[nmax] ;

int n ;

int main()
{
    freopen( "aria.in" , "r" , stdin ) ;
    freopen( "aria.out" , "w" , stdout ) ;

    scanf( "%d" , &n ) ;
    for ( int i = 1 ; i <= n ; i ++ )
        scanf( "%lf %lf" , &v[i].x , &v[i].y ) ;

    v[0].x = v[n].x ;
    v[0].y = v[n]. y ;
    double sum = 0 ;
    for ( int i = 1 ; i <= n  ; i ++ )
        sum += ( v[i-1].x * v[i].y ) - ( v[i].x * v[i-1].y ) ;
    sum /= 2.0 ;

    cout << setprecision(6) << sum ;

    return 0;
}