#include <iostream>
#include <fstream>
#define maxn 100009
using namespace std;
struct punct{int x;int y;}v[maxn];
int main()

{int x,y,a,b,i,n,sum=0;
fstream f("aria.in",ios::in);
fstream g("aria.out",ios::out);
f>>n;
f>>v[1].x>>v[1].y;
for(i=2;i<=n;i++)
{f>>v[i].x>>v[i].y;
sum=sum+((v[i-1].x*v[i].y)-(v[i].x*v[i-1].y))/2;
}

sum=sum+((v[n].x*v[1].y)-(v[1].x*v[n].y))/2;

g<<sum;

}