#include <fstream>
#include <math.h>
#include <vector>
#include <set>
#include <algorithm>
#include <string>
#include <map>
#include <iomanip>
//#include <iostream>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
int main(){
    double x[100005], y[100066], sol = 0;
    int n;
    cin >> n;
    for(int i = 0; i < n; i++)
        cin >> x[i] >> y[i];
    x[n] = x[0];
    y[n] = y[0];
    for(int i = 0; i < n; i++)
    {
        sol += (x[i] * y[i + 1] - y[i] * x[i + 1]);
    }
    cout << sol / 2.0;
}