#include<string>
#include<cmath>
#include<iostream>
#include<algorithm>
#include<iomanip>
using namespace std;
int main()
{
    int n;
    cin >> n;
    long double mas_x[100000];
    long double mas_y[100000];
    for (int i = 1; i <= n; i++)
    {
        cin >> mas_x[i] >> mas_y[i];
    }
    long double res = 0;
    mas_x[n+1] = mas_x[1];
    mas_y[n+1] = mas_y[1];
    for (int i = 1; i <= n; i++)
    {
        res += ((mas_y[i + 1] + mas_y[i])*(mas_x[i + 1] - mas_x[i]));
    }
	cout<<(abs(res) / 2.0);
    return 0;
}