#include <iostream>
#include <fstream>
using namespace std;
ifstream f("arii.in");
ofstream g("arii.out");
struct puncte
{
    int x;
    int y;
}p[30];
int i,j,n,aria;
int main()
{
f>>n;
for(i=1;i<=n;i++)
    f>>p[i].x>>p[i].y;
aria=0;
for(i=1;i<n;i++)
{
    aria+=p[i].x*p[i+1].y-p[i+1].x*p[i].y;
}
    aria+=p[n].x*p[1].y-p[1].x*p[n].y;
    aria/=2;
    g<<aria;
}