#include <iostream>
#include <fstream>
#include <queue>

using namespace std;

ifstream fin("arie.in");
ofstream fout("arie.out");

struct punct {
double x, y;
};

queue<punct> coada;

double aria (punct p1, punct p2, punct p3) {
    return (p1.x * p2.y + p2.x * p3.y + p3.x * p1.y - p1.y * p2.x - p2.y * p3.x - p3.y * p1.x) / 2;
}

int main()
{
    int n;
    double sum;
    punct control, save, A, B;
    control.x = control.y = 0;

    fin >> n >> save.x >> save.y;

    A = save;

    n--;
    while (n) {
        fin >> B.x >> B.y;
        sum += aria(control, A, B);
        A=B;
        n--;
    }
    sum += aria(control, B, save);

    fout << sum;

    return 0;
}