#include <fstream>
#include <iomanip>
std::ifstream cin("aria.in");
std::ofstream cout("aria.out");
#define maxn 100005

int n;
struct {
	long double x,y;
}pct[maxn];
long double ans;

int main()
{
	cin>>n;
	for(int i=1;i<=n;i++)
		cin>>pct[i].x>>pct[i].y;
	pct[n+1]=pct[1];
	for(int i=1;i<=n;i++)
		ans+=(pct[i].x*pct[i+1].y- pct[i+1].x*pct[i].y);
	ans/=2;
	cout<<std::fixed<<std::setprecision(6)<<ans;
}