#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 modp=1999999973;
const int modx=998244353;

 
ifstream fin("aria.in");
ofstream fout("aria.out"); 

int n;
pair<int,int>point[100005];
 
int cross(int x,int y,int a,int b,int c,int 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());
	fin >> n;
	for(int i=1;i<=n;i++) fin >> point[i].fr >> point[i].sc;    
	++n;
	point[n].fr=point[1].fr,point[n].sc=point[1].sc;
	int sum=0;
	for(int i=1;i<=(n-1);i++){ 
		sum+=cross(0,0,point[i].fr,point[i].sc,point[i+1].fr,point[i+1].sc);
	}
	fout << abs(sum/2);
}