#include <iostream>
#include <fstream>
#include <iomanip>

#define NMAX 100005

using namespace std;

struct punct
{
    long double x, y;
};

long double determinant(punct A, punct B)
{
    return A.x * B.y - A.y * B.x;
}

int N;
long double arie;
punct A[NMAX], o;

void citire()
{
    cin >> N;

    for (int i = 1; i <= N; ++i)
        cin >> A[i].x >> A[i].y;
    A[N + 1] = A[1];
    o.x = o.y = 0;
}

void rezolvare()
{
    long double det;

    for (int i = 1; i <= N; ++i)
    {
        det = determinant(A[i], A[i + 1]);
        arie += det / 2;
    }

    if (arie < 0)
        arie = -arie;

    cout << setprecision(5) << arie;
}

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

    citire();
    rezolvare();

    return 0;
}