#define _CRT_SECURE_NO_WARNINGS
#include <string>
#include <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <queue>
#include <iostream>
#include <sstream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstring>
#include <cctype>
#include <cassert>
#include <limits>
#include <functional>
#include <stack>
#include <array>
#include <fstream>

#define REP(i,n) for(int (i) = 0;(i) < (n) ; ++(i))
#define REPA(a,i,n) for(int (i) = (a) ; (i) < (n) ; ++(i))
#if defined(_MSC_VER)||__cplusplus > 199711L
#define AUTO(r,v) auto r = (v)
#else
#define AUTO(r,v) typeof(v) r = (v)
#endif
#define ALL(c) (c).begin() , (c).end()
#define EACH(it,c) for(AUTO(it,(c).begin());it != (c).end();++it)
#define LL long long
#define int LL
#define INF  99999999
#define DEV 1000000007
#define QUICK_CIN ios::sync_with_stdio(false); cin.tie(0);
using namespace std;

int n;
vector<int> a;

signed main()
{
	QUICK_CIN;
	//ifstream cin("debug.txt");
	//ofstream cout("result.txt");

	cin >> n;

	REP(i, n){
		int x, y;
		cin >> x >> y;

		a.push_back(x + 4 * y);
	}

	int maxi = *std::max_element(a.begin(), a.end());

	int ans(0);

	REP(i, n){
		int temp;
		temp = maxi - a[i];
		if (temp % 2){
			cout << -1 << endl;
			return 0;
		}
		ans += temp;
	}

	cout << ans / 2 << endl;

	return 0;
}