#include <algorithm>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <ctime>
#include <iostream>
#include <sstream>
#include <functional>
#include <map>
#include <string>
#include <cstring>
#include <vector>
#include <queue>
#include <stack>
#include <deque>
#include <set>
#include <list>
#include <numeric>
using namespace std;
const double PI = 3.14159265358979323846;
const double EPS = 1e-12;
const int INF = 1<<25;
typedef pair<int,int> P;
typedef long long ll;
typedef unsigned long long ull;


int main(){
	int n;
	cin>>n;
	vector<ll> d(n);
	ll mx = 0;
	for(int i = 0; i < n; i++){
		int a, b;
		cin>>a>>b;
		d[i] = a+4*b;
		mx = max(mx, d[i]);
	}
	bool ok = true;
	ll res = 0;
	for(int i = 0; i < n; i++){
		ll dd = mx-d[i];
		if(dd&1) ok = false;
		res += dd/2;
	}
	if(!ok) cout<<-1<<endl;
	else cout<<res<<endl;
	return 0;
}