#include <iostream>
#include <fstream>
#include <iomanip>
#define NUM 100005
struct Punct
{
    double x, y;
};
Punct v[NUM];
int n;
long double sum;
using namespace std;
long double rez(long double arie)
{
    if(arie < 0)
        return  - arie / 2;
    return arie / 2;
}
int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");
    f >> n;
    for(int i = 1; i <= n; ++i)
        f >> v[i].x >> v[i].y;
    v[0] = v[n];
    v[n + 1] = v[1];
    for(int i = 1; i <= n; ++i)
        sum += v[i].x * (v[i + 1].y - v[i - 1].y);
    g << fixed << setprecision(5) << rez(sum);
    f.close();
    g.close();
}