#include <iostream>
#include <fstream>
#include <algorithm>
#include <vector>

#define xx first
#define yy second

using namespace std;

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

const int NMAX = 1e5 + 100;

int N;
typedef pair <double, double> pct;
pct P[NMAX];

double Sign(pct A, pct B)
{
    return A.xx * B.yy - A.yy * B.xx;
}

int main()
{
    fin >> N;
    for(int i = 1; i <= N; i++)
        fin >> P[i].xx >> P[i].yy;
    P[N + 1] = P[1];

    double Arie = 0;
    for(int i = 2; i <= N + 1; i++)
        Arie += Sign(P[i - 1], P[i]);

    fout.precision(5);
    fout << fixed << (Arie / 2.0);
    return 0;
}