#include <fstream>
#include <iostream>
#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;

struct pontS
{
    ld x, y;
};

pontS v[NLIM];

int main()
{
    fin >> N;
    for( int i = 0; i < N; ++i )
        fin >> v[i].x >> v[i].y;
    v[N] = v[0];

    ld res = 0;
    for( int i = 0; i < N; ++i )
    {
        res += v[i].x * v[i+1].y  - v[i+1].x * v[i].y;
    }
    res /= 2;

    fout << setprecision( 50 ) <<  res << "\n";

    return 0;
}