//g++ 2.cpp -std=c++14 -I .
#include <bits/stdc++.h>
using namespace std;

#include <atcoder/all>
using namespace atcoder;

using ll = long long;
using ld = long double;

using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using vld = vector<ld>;
using vvld = vector<vld>;

#define fi first
#define se second
#define pb push_back
#define all(a) a.begin(),a.end()
#define rep(i,start,end) for(ll i=start;i<(ll)(end);i++)
#define per(i,start,end) for(ll i=start;i>=(ll)(end);i--)

int main(){
  ios::sync_with_stdio(false);
  cin.tie(nullptr);

  ll ans=0;

  int n;
  cin>>n;

  vector<pair<ll,ll>> v(n);
  rep(i,0,n){
    cin>>v[i].fi>>v[i].se;
  }

  ll a,b,c,d,e,f;
  a=v[0].fi,b=v[0].se;

  rep(i,0,n-2){
    c=v[i+1].fi,d=v[i+1].se;
    e=v[i+2].fi,f=v[i+2].se;

    ans+=abs((a-e)*(d-f)-(c-e)*(b-f));
  }

  cout<<ans<<endl;
}