#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (n); i++)
typedef long long ll;
typedef pair<int, int> pii;
typedef vector<ll> vll;
typedef vector<int> vi;
typedef vector<vector<int>> vvi;
typedef vector<vector<ll>> vvll;
const ll inf = 1e16;
const ll md = 1000000007;

int main() {
  int n;
  cin>>n;
  vll a(n);
  vll b(n);
  rep(i,n){
    cin>>a[i];
    cin>>b[i];
  }
  rep(i,n) if(a[0]&1!=a[i]&1){
    puts("-1");
    return 0;
  }
  ll mx=0;
  rep(i,n) mx=max(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;
}