/*
ID: Sho10
LANG: C++
TASK:
*/
#include <bits/stdc++.h> //JuniorMonster a.k.a Sho10
#define ll long long int
#pragma GCC optimize("O3")
#pragma GCC optimize("Ofast")
#define all(a) (a).begin(), (a).end()
#define sz size
#define f first
#define s second
#define pb push_back
#define er erase
#define in insert
#define mp make_pair
#define pi pair
#define rc(s) return cout<<s,0
#define endl '\n'
#define mod 1000000007
#define PI 3.14159265359
#define CODE_START  ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
using namespace std;
ll n;
pair<double,double>a[100005];
double ans;
int32_t main(){
CODE_START;
ifstream cin("aria.in");
ofstream cout("aria.out");
cin>>n;
for(ll i=0;i<n;i++)
{
    cin>>a[i].f>>a[i].s;
}
a[n].f=a[0].f;
a[n].s=a[0].s;
for(ll i=0;i<n;i++)
{
    ans=ans+a[i].f*(a[i+1].s)-a[i].s*a[i+1].f;
}
cout<<fixed<<setprecision(9);
if(ans<0){
    ans=ans*(-1.0);
}
ans=ans/double(2.0);
cout<<ans<<endl;
}