#include<bits/stdc++.h>
#include "testlib.h"

using namespace std;
using pi=pair<int,int>;

int cp(pi a,pi b){
  return a.first*b.second-a.second*b.first;
}

pi vm(pi a,pi b){
  return make_pair(a.first-b.first,a.second-b.second);
}

int main(int argc, char* argv[]){
  registerValidation(argc, argv);
  int n=inf.readInt(3,30);inf.readEoln();
  vector<pi> vp(n);
  for(int i=0;i<n;i++){
    vp[i].first=inf.readInt(-10000,10000);inf.readSpace();
    vp[i].second=inf.readInt(-10000,10000);inf.readEoln();
  }
  for(int i=0;i<n;i++){
    ensuref(cp(vm(vp[(i+1)%n],vp[i]),vm(vp[(i+2)%n],vp[(i+1)%n]))<0,"the points aren't clockwise");
  }
  int res=0;
  for(int i=2;i<n;i++){
    res+=abs(cp(vm(vp[i-1],vp[0]),vm(vp[i],vp[0])));
  }
  cout << res << '\n';
  inf.readEof();
  return 0;
}