#include<bits/stdc++.h>
using namespace std;
using uint = unsigned int;
using ll = long long;
using ull = unsigned long long;
template<class T> using V = vector<T>;
template<class T> using VV = V<V<T>>;
template<class T> using VVV = V<VV<T>>;
template<class T> using VVVV = VV<VV<T>>;
#define rep(i,n) for(ll i=0ll;i<n;i++)
#define REP(i,a,n) for(ll i=a;i<n;i++)
const long long INF = (1LL << 60);
const long long mod99 = 998244353;
const long long mod107 = 1000000007;
const long long mod = mod99;
#define eb emplace_back
#define pb eb
#define be(v) (v).begin(),(v).end()
#define all(i,v) for(auto& i : v)
#define all2(i,j,v) for(auto& [i,j] : v)
template<class T, class U> void chmin(T& t, const U& u) { if (t > u) t = u; }
template<class T, class U> void chmax(T& t, const U& u) { if (t < u) t = u; }

// cin.tie(nullptr);
// ios::sync_with_stdio(false);
// cout << fixed << setprecision(20);

void solve(){
  V<ll>v;
  ll n;
  cin >> n;
  rep(i,n){
  	ll a,b;
  	cin >> a >> b;
  	v.eb(a+4*b);
  }
  sort(be(v), greater<ll>());
  ll ans = 0;
  rep(i, n){
  	if(v[0]%2!=v[i]%2) ans = -INF;
  	else ans += (v[0]-v[i])/2;
  }
  if(ans < 0) ans = -1;
  cout << ans << endl;
}





int main(){
  cin.tie(nullptr);
  ios::sync_with_stdio(false);
  int t=1;
  // cin >> t;
  rep(i,t) solve();
}