#include <iostream>
#include <fstream>
#include <cmath>
#include <cstdio>
#include <iomanip>

using namespace std;

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


int n;
long double s=0;


struct coord{

    long double x,y;
}a[100005];

int main()
{
    f>>n;
    for(int i=0;i<n;i++)
    {
        f>>a[i].x>>a[i].y;
    }
    for(int i=0;i<n-1;i++)
    {
        s+=((a[i].y + a[(i + 1) ].y) * (a[(i + 1)].x - a[i].x))/2;

    }
    s+=((a[n-1].y+a[(0) ].y) * (a[(0)].x - a[n-1].x))/2;
    g<<std::fixed;
    g<<std::setprecision(5)<<abs(s);
    return 0;
}