#include <bits/stdc++.h>
#define ll long long
#define all(a) (a).begin(), (a).end()
//#pragma GCC optimize("O3")
//#pragma GCC optimize("Ofast")
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
#define sz() size()
#define fr first
#define sc second
#define pb push_back
#define er erase
#define in insert
#define pi pair<int,int>
#define pii pair<pair<int,int>,int>
#define mp make_pair
//#define int long long
#define rc(s) return cout<<s,0
#define rcc(s) cout<<s,exit(0)
using namespace std;

const int mod=1e9+7;
const int modx=998244353;
const int per=666013;

int n;
double ans;
pair<double,double>a[100005];

double cross(double x,double y,double a,double b,double c,double d){
    return (a-x)*(d-y)-(c-x)*(b-y);
}

int32_t main(){
   // ios_base::sync_with_stdio(false);cin.tie(0);cerr.tie(0);cout.tie(0);
    srand(chrono::steady_clock::now().time_since_epoch().count());
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    fin >> n;
    for(int i=1;i<=n;i++){
        fin >> a[i].fr >> a[i].sc;
    }
    a[n+1].fr=a[1].fr,a[n+1].sc=a[1].sc;
    for(int i=1;i<=n;i++){
        ans+=(0.5*cross(0.0,0.0,a[i].fr,a[i].sc,a[i+1].fr,a[i+1].sc));
    }
    fout << abs(ans);
}