#include <algorithm>
#include <cstdio>
#include <iostream>
#include <string>
#include <vector>
#include <map>
#define rep(i, n) for (int i=0; i<n; ++i)
#define FOR(i, a, b) for (int i=a; i<b; ++i)
#define pb(x) push_back(x)
#define ll long long
#define ul unsigned long long
#define debug(x) cout  << #x << " : " << x << endl;
using namespace std;

const int Inf =1500000000;
int main () {
  int n;
  cin >> n;
  int c[n];
  rep(i, n) {
    int a, b;
    cin >> a >> b;
    c[i] = a + 4 * b;
  } 
  sort(c, c + n);
  int max = c[n - 1];
  int p = c[n - 1] % 2;
  ll cnt = 0;
  rep(i, n - 1) {
    if (p != c[i] % 2) {
      cnt = -1;
      break;
    } else {
      cnt += (max - c[i]) / 2; 
    }
  }
  cout << cnt << endl;
	return 0;
}