#include <iostream>
#include <fstream>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int i,j,n,k,s=0;
struct gogu
{
    float x,y;
}v[1000010];
int main()
{
    f>>n;
    for(i=1;i<=n;i++)
    {
        f>>v[i].x>>v[i].y;
    }
    for(i=1;i<n;i++)
    {
        s=s+(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
    }
    s=s+(v[n].x*v[1].y-v[1].x*v[n].y);
    g<<s/2;
}