#include<bits/stdc++.h>
#include<fstream>
using namespace std;
ifstream fi("aria.in");
ofstream fo("aria.out");
struct p{
	double  x,y;
}v[100005];
int main(){
	int n,s;
	fi>>n;
	for(int i=1;i<=n;i++){
		fi>>v[i].x>>v[i].y;
	}
	v[n+1]=v[1];
	s=0;
	for(int i=1;i<=n;i++)
	   s+=(v[i].x*v[i+1].y-v[i+1].x*v[i].y);
	   fo<<setprecision(6)<<abs(s/2);
}