#include <map>
#include <set>
#include <list>
#include <cmath>
#include <queue>
#include <stack>
#include <cstdio>
#include <string>
#include <vector>
#include <complex>
#include <cstdlib>
#include <cstring>
#include <numeric>
#include <sstream>
#include <iostream>
#include <algorithm>
#include <functional>
 
#define mp       make_pair
#define pb       push_back
#define all(x)   (x).begin(),(x).end()
#define rep(i,n) for(int i=0;i<(n);i++)
 
using namespace std;
 
typedef    long long          ll;
typedef    unsigned long long ull;
typedef    vector<bool>       vb;
typedef    vector<int>        vi;
typedef    vector<vb>         vvb;
typedef    vector<vi>         vvi;
typedef    pair<int,int>      pii;
 
const int INF=1<<29;
const double EPS=1e-9;
 
const int dx[]={1,0,-1,0},dy[]={0,-1,0,1};
int main(int argc, char const *argv[]) {
	
	int N;
	std::vector<int> haba;
	cin >> N;
	for (int i = 0; i < N; ++i) {
		int temp1, temp2;
		cin >> temp1 >> temp2;
		haba.push_back(temp1 + temp2 * 4);
	}

	int ans = 0;
	int max = 0;
	int odd = haba[0] % 2;
	for (int i = 0; i < N; ++i) {
		if (odd != haba[i] % 2) {
			cout << "-1" << endl;
			return 0;
		}
		max = std::max(max, haba[i]);
	}

	for (int i = 0; i < N; ++i) {
		ans += (max - haba[i]) / 2;
	}
	cout << ans << endl;



	return 0;
}