#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <set>
#include <map>
#include <algorithm>
#include <functional>
#include <limits>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <climits>
#include <complex>

using namespace std;

#define INF (INT_MAX/3)
#define PI (2*acos(0.0))
#define EPS (1e-8)

typedef long long ll;
typedef unsigned long long ull;

int main(){
  ios_base::sync_with_stdio(0);
  int N, a, b, m;  cin >> N;
  vector<int> L(N);

  for(int i = 0; i < N; i++){
    cin >> a >> b;
    L[i] = a + b * 4;
    m = max(L[i], m);
    if(i >= 1 && L[i] % 2 != L[i-1] % 2) { cout << -1 << endl; return 0;}
  }

  ull ans = 0;
  for(int i = 0; i < N; i++){
    ans += m - L[i];
  }
  cout << ans / 2 << endl;
  return 0;
}