#include <bits/stdc++.h>
using namespace std;
using ll=long long;
using ull=unsigned long long;
#define rep(i,n) for(int i=0; i<(n); i++)

int N;
ll X[30], Y[30];

ll triangle(int a,int b,int c){
  return abs((X[b]-X[a])*(Y[c]-Y[a])-(X[c]-X[a])*(Y[b]-Y[a]));
}

int main(){
  cin>>N;
  rep(i,N) cin>>X[i]>>Y[i];
  ll ans=0;
  for(int i=2; i<N; i++) ans += triangle(0,i-1,i);
  cout<<ans<<"\n";
  return 0;
}