#include <iostream>
#include <fstream>
#include <iomanip>
#define N 100001
// 1 x0 y0
// 1 x1 y1
// 1 x2 y2

template <typename T> struct point
{
    T x,y;
    point():x(0),y(0){}
    point(T x, T y):x(x),y(y){}
};

point<long double> p[N];

template <typename T>
T area2(point<T> p0, point<T> p1, point<T> p2)
{
   T ret = +p1.x*p2.y - p2.x*p1.y 
           -p0.x*p2.y - p2.x*p0.y 
           +p0.x*p1.y - p1.x*p0.y;
   return ret;
}


int main ()
{
    std::ifstream fin("aria.in");
    std::ofstream fout("aria.out");
    int n,x,y;
    long double total=0;
    
    fin>>n;
    for(int i=0;i<n;i++)
    {
        fin>>p[i].x>>p[i].y;
    }
    p[n]=p[0];
    point<long double> zero(0,0);

    for(int i=0;i<n;i++)
    {
        long double area = area2(zero, p[i], p[i+1]);
        total+=area/2.0;
    }
    fout<<std::setprecision(15)<<total;
    return 0;
}