#include <bits/stdc++.h>

using namespace std;
ifstream f ("aria.in");
ofstream g ("aria.out");
const int L = 1e5 + 5;
struct coordonate
{
    int x, y;
};
coordonate puncte[L];
int main()
{
    int n, s = 0;
    f >> n;
    for (int i = 1; i <= n * 2; i++)
        f >> puncte[i].x >> puncte[i].y;
    for (int i = 1; i <= n; i++)
    {
        if (i == n)
        {
            puncte[i+1].x = puncte[1].x;
            puncte[i+1].y = puncte[1].y;
        }
        s += puncte[i].x * puncte[i+1].y - puncte[i+1].x * puncte[i].y;
    }
    g << s / 2;
    return 0;
}