#include <bits/stdc++.h>



using namespace std;

ifstream f("aria.in");

ofstream g("aria.out");

int i,n;

long double a[100001],b[100001];

long double sum;

int main(int argc, char *argv[]) {

	f>>n;

	f>>a[1]>>b[1];

	for(i=2;i<=n;i++)

	{

		f>>a[i]>>b[i];

		sum+=(a[i-1]*b[i]-a[i]*b[i-1])*0.5;

	}

	sum+=(a[n]*b[1]-a[1]*b[n])*0.5;

	g<<fixed<<setprecision(6)<<sum;

}