#include <algorithm>
#include <cfloat>
#include <climits>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <functional>
#include <iostream>
#include <map>
#include <memory>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <utility>
#include <vector>
 
using namespace std;
 
#define sz size()
#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define all(c) (c).begin(), (c).end()
#define rep(i,a,b) for(long long i=(a);i<(b);++i)
#define clr(a, b) memset((a), (b) ,sizeof(a))
 
#define MOD 1000000007

int main(){
  int n;
  cin>>n;
  map<int, int> ma;
  vector<pair<int,int> > v; 
  rep(i,0,n){
    int a,b;
    cin>>a>>b;
    v.pb(mp(a,b));
    int mn = a+4*b;
    ma[mn]++;
  }
  map<int,int>::iterator itr = ma.begin();
  int c = 0;
  int d = -1;
  while(itr!=ma.end()){
    c += (*itr).se;
    if(c==n){
      d = (*itr).fi;
      break;
    }
    itr++;
  }
  int ans = 0;
  rep(i,0,v.sz){
    if((v[i].fi+v[i].se*4)%2!=d%2){
      cout << -1 << endl;
      return 0;
    }
    ans += (d-4*v[i].se-v[i].fi)/2;
  }
  cout << ans << endl;
  return 0;
}