#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <cmath>
#include <iomanip>
#include <stack>
#include <queue>
#include <numeric>
#include <map>
#include <unordered_map>
#include <set>
#include <fstream>
#include <chrono>
#include <random>
#include <bitset>
//#include <atcoder/all>
#define rep(i,n) for(int i=0;i<(n);i++)
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) ((int)(x).size())
#define pb push_back
using ll = long long;
using namespace std;
template<class T>bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T>bool chmin(T &a, const T &b) { if (b<a) { a=b; return 1; } return 0; }
ll gcd(ll a, ll b) {return b?gcd(b,a%b):a;}
ll lcm(ll a, ll b) {return a/gcd(a,b)*b;}

int main(){
  int N; cin >> N;
  vector<int> A(N),B(N);
  vector<int> chk(2,0);
  rep(i,N){
    cin >> A[i] >> B[i];
    chk[(A[i]+B[i]*4)%2]=1;
  }
  if(chk[0]&&chk[1]) cout << -1 << endl;
  else{
    int mx=0;
    rep(i,N) chmax(mx,A[i]+B[i]*4);
    ll ans = 0;
    rep(i,N) ans += (mx-(A[i]+B[i]*4))/2;
    cout << ans << endl;
  }
  return 0;
}