#include <iostream>
#include <fstream>
#include <vector>
#include <stack>
#include <sstream>
#include <cmath>
#include <iomanip>

#define ld long double

using namespace std;

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

const int NLIM = 1e5 + 10;

int N;

int orient( int x1, int yy1, int x2, int y2, int x3, int y3 )
{


    int v = ( y2 - yy1 )*( x3 - x2 ) - ( y3 - y2 )*( x2 - x1 );

    /*/
    cout << x1 << " " << y1 << "\n";
    cout << x2 << " " << y2 << "\n";
    cout << x3 << " " << y3 << "\n";
    cout << v << "\n-----------\n";
    //*/
    if( v > 0 )
        return -1;
    else if( v < 0 )
        return 1;
    else
        return 0;

}

int x1, yy1;
int lx, ly;
int px = 0;
int py = 0;

ld res = 0;




int main()
{
    fin >> N;

    fin >> lx >> ly;
    for( int i = 1; i < N; ++i )
    {
        int x, y;
        fin >> x >> y;

        ld a = sqrt( ( x - px ) * ( x - px ) + ( y - py ) * ( y - py ) );
        ld b = sqrt( ( lx - px ) * ( lx - px ) + ( ly - py ) * ( ly - py ) );
        ld c = sqrt( ( lx - x ) * ( lx - x ) + ( ly - y ) * ( ly - y ) );
        ld T;
        ld s = ( a + b + c ) / 2;
        T = sqrt( s*( s - a )*( s - b )*( s - c ) );

        //cout << a << " " << b << " " << c << " " << T << "\n";


        int o = orient( px, py, lx, ly, x, y );
        if( o == 1 )
        {
            res += T;
            //cout << "-\n";
        }
        else if( o == -1 )
        {
            res -= T;
           // cout << "+\n";
        }



        if( i == 1 )
        {
            x1 = lx;
            yy1 = ly;
        }
        lx = x;
        ly = y;
    }

    int x, y;
    x = x1;
    y = yy1;
    ld a = sqrt( ( x - px ) * ( x - px ) + ( y - py ) * ( y - py ) );
    ld b = sqrt( ( lx - px ) * ( lx - px ) + ( ly - py ) * ( ly - py ) );
    ld c = sqrt( ( lx - x ) * ( lx - x ) + ( ly - y ) * ( ly - y ) );
    ld T;
    ld s = ( a + b + c ) / 2;
    T = sqrt( s*( s - a )*( s - b )*( s - c ) );
   // cout << a << " " << b << " " << c << " " << T << "\n";
    int o = orient( px, py, lx, ly, x, y );
    if( o == 1 )
    {
        res += T;
       // cout << "-\n";
    }
    else if( o == -1 )
    {
        res -= T;
        //cout << "+\n";
    }

    fout << setprecision( 5 ) << res << "\n";

    return 0;
}