#include <algorithm>
#include <iostream>
#include <iomanip>
#include <map>
#include <numeric>
#include <set>
#include <sstream>
#include <string>
#include <vector>
#include <cmath>
using namespace std;

#define FOR(i,s,e) for (int i = int(s); i < int(e); i++)
#define FORIT(i,c) for (typeof((c).begin()) i = (c).begin(); i != (c).end(); i++)
#define ISEQ(c) (c).begin(), (c).end()

int SURP = 1000000007;

int main(){
	int N;
	cin >> N;
	int width[N];
	int maxw = 0;

	FOR(i,0,N){
		int ai,bi;
		cin >> ai >> bi;
		width[i] = ai + 4 * bi;
		if(maxw < width[i]) maxw = width[i];
	}

	int cnt = 0;

	FOR(i,0,N){
		int diff = maxw - width[i];
		if (diff%2 != 0){
			cnt = -1;
			break;
		}else{
			cnt += diff/2;
		}
	}

	cout << cnt << endl;
	return 0;
}