#include <iostream>
#include <fstream>
#include <vector>
#include <iomanip>

using namespace std;

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

struct Pos
{
    long double x;
    long double y;
};

int main()
{
    int N;
    input >> N;
    vector<Pos> points(N);
    for(int i = 0; i < N; i++){
        input >> points[i].x;
        input >> points[i].y;
    }

    long double area = 0;

    for(int i = 1; i < N; i++){
        area += (points[i-1].x * points[i].y - points[i].x * points[i-1].y)/2;
    }
    area += (points[N-1].x * points[0].y - points[0].x * points[N-1].y)/2;
    output << fixed << setprecision(6) << area << endl;
    return 0;
}