#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;
  vector<int> v;
  int mx = 0;
  rep(i,0,n){
    int a,b;
    cin>>a>>b;
    v.pb(a+4*b);
    mx = max(mx,a+4*b);
  }
  int ans = 0;
  rep(i,0,v.sz){
    if(mx%2!=v[i]%2){
      cout << -1 << endl;
      return 0;
    }
    ans += (mx-v[i])/2;
  }
  cout << ans << endl;
  return 0;
}