#include <fstream>
#include <iomanip>

using namespace std;
const int NMAX = 100002;
using ld = long double;

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

struct puncte {
    ld x, y;
}v[NMAX];
int main()
{
    int n;
    cin >> n;
    for(int i = 1; i <= n; i++)
        cin >> v[i].x >> v[i].y;
    v[n + 1].x = v[1].x;
    v[n + 1].y = v[1].y;
    ld ans = 0;
    for(int i = 1; i <= n; i++)
        ans += (v[i].x * v[i + 1].y - v[i + 1].x * v[i].y);
    cout << setprecision(5) << fixed << (ans / 2.0);
    return 0;
}