#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;
long double ans;

struct Point{
    long double x,y;
}a[100005];


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");
    fout << fixed << setprecision(9);
    fin >> n;
    for(int i=1;i<=n;i++){
        fin >> a[i].x >> a[i].y;
    }
    a[n+1].x=a[1].x,a[n+1].y=a[1].y;
    for(int i=1;i<=n;i++){
        ans+=a[i].x*a[i+1].y-a[i].y*a[i+1].x;
    }
    if(ans<0) ans*=(-1.0);
    ans/=(2.0);
    fout << ans;
}