#include <iostream>
#include <fstream>
#include <cmath>
#define x first
#define y second

using namespace std;

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

pair <double, double> V[100010];

int main()
{
    int N, i;
    double Ans = 0;

    in >> N;

    for (i = 1; i <= N; i ++)
        in >> V[i].x >> V[i].y;

    V[N + 1] = V[N];

    for (i = 1; i <= N; i ++)
        Ans += (V[i].x * V[i + 1].y - V[i + 1].x * V[i].y);

    out << fabs (Ans / 2.0);

    return 0;
}