#include<fstream>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
const int N = 100005;
int x[N],y[N];
long long A;
int n;
int main(){
	int i;
	in>>n;
	for(i=1;i<=n;i++) in>>x[i]>>y[i];
	x[n+1]=x[1];
	y[n+1]=y[1];
	for(i=1;i<=n;i++) A+=x[i]*y[i+1]-x[i+1]*y[i];
	out<<A/2;
	return 0;
}