#include <fstream>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int n, x[100010], y[100010], i, j, k, a;
int main(){
    f>>n;
    for(i=1; i<=n; i++)
		f>>x[i]>>y[i];
	f.close();
	for(i=1; i<n; i++)
		a=a+(x[i]*y[i+1]-x[i+1]*y[i])/2;
	a=a+(x[n]*y[1]-x[1]*y[n])/2;
	if(a<0)
		a=-a;
	g<<a<<"\n";
	g.close();
    return 0;
}