#include <fstream>
#include <iostream>

using namespace std;

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

const int Max = 100007;
int n;

struct coordonate
{
    int x, y;
}v[Max];

int alan()
{
    double s = 0;

    for(int i = 1; i <= n; ++i)
    {
        s+= v[i].x * v[i + 1].y - v[i + 1].x * v[i].y;
    }

    return s / 2;
}

int main()
{
    in >> n;
    for(int i = 1; i <= n; ++i)
        in >> v[i].x >> v[i].y;
    v[n + 1].x = v[1].x;
    v[n + 1].y = v[1].y;
    out << alan() << "\n";

    return 0;
}