#include <iostream>
#include <vector>
#include <string>
#include <array>
#include <queue>
#include <deque>
#include <set>
#include <list>
#include <map>
#include <stack>
#include <utility>
#include <algorithm>
#include <numeric>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cctype>
#include <cstring>
#include <climits>
#include <bitset>
#include <random>
#include <functional>
#include <sstream>
#include <iomanip>

using namespace std;

#define _CRT_SECURE_NO_WARNINGS
#define REP(i, n) for(int i=0; i<(n); i++)
#define FOR(i, m, n) for(int i=(m);i<(n);i++)
#define SZ(x) ((int)(x).size())
#define ALL(x) (x).begin(),(x).end()
#define SORT(x) sort((x).begin(),(x).end())
#define REVE(x) reverse((x).begin(),(x).end())
#define MP make_pair
#define PB push_back

typedef vector<int> VI;
typedef vector<string> VS;
typedef vector<vector<int>> VVI;
typedef pair<int, int> PII;
typedef long long LL;

int main() {
	int n;
	cin >> n;
	VI a(n), b(n), c(n);
	REP(i, n)cin >> a[i] >> b[i];
	REP(i, n) {
		c[i] = a[i] + b[i] * 4;
	}
	SORT(c);
	bool flag = true;
	REP(i, n - 1)if ((c[i + 1] - c[i]) % 2 == 1)flag = false;
	
	if (!flag) {
		puts("-1");
	}
	else {
		int cnt = 0;
		REP(i, n - 1) {
			cnt += (c[n - 1] - c[i]) / 2;
		}
		cout << cnt << endl;
	}
}