#include <iostream>
#include <fstream>

using namespace std;

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

long double aria;
struct {long double x,y;} p,last_p,first_p;
int n;

int main()
{
    f>>n;
    f>>last_p.x>>last_p.y;
    first_p=last_p;
    for(--n;n;--n)
    {
        f>>p.x>>p.y;
        aria+=last_p.x*p.y-p.x*last_p.y;
        last_p=p;
    }
    p=first_p;
    aria+=last_p.x*p.y-p.x*last_p.y;
    g<<aria/2;
    return 0;
}