#include<fstream>
#include<cstdio>
#include<set>
#include<stack>
#include<vector>
#include<algorithm>
#define FOR(a,b,c) for(int a=b;a<=c;++a)
#include<cstring>
#include<bitset>
#include<cmath>
#include<iomanip>
#include<queue>
#define f cin
#define g cout
#define mp make_pair
#define pb push_back
#define x first
#define y second
#define ll long long
#define inf (1<<30)
#define base 256
#define ba 255
#define N 100100
#define EPS 1e-12
#define mod  666013
#define inu "aria.in"
#define outu "aria.out"
using namespace std;
ifstream f(inu);
ofstream g(outu);
//int dx[]={0,0,0,1,-1};
//int dy[]={0,1,-1,0,0};
pair<int,int> v[N];
int n;
long long area;
int main ()
{
	f>>n;
	FOR(i,1,n)
	{
		f>>v[i].x>>v[i].y;
	}
	v[n+1].x=v[1].x;
	v[n+1].y=v[1].y;
	FOR(i,1,n)
	{
		area+=1LL*v[i].x*v[i+1].y-1LL*v[i].y*v[i+1].x;
	}
	if(area&1)
	g<<area/2<<".50000";
	else
	g<<fixed<<setprecision(5)<<area/2;
    return 0;
}