import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int odd = 0;
    int even = 0;
    int max = 0;
    int[] a = new int[N];
    int[] b = new int[N];
    for(int i = 0; i < N; i++) {
      a[i] = sc.nextInt();
      b[i] = sc.nextInt();
      if(a[i] % 2 == 0) {
        even++;
      } else {
        odd++;
      }
      max = Math.max(max, a[i] + 4 * b[i]);
    }
    int ans = 0;
    if(even == 0 || odd == 0) {
      for(int i = 0; i < N; i++) {
        ans += ((max - a[i] - 4 * b[i]) / 2);
      }
    } else {
      ans = -1;
    }
    System.out.println(ans);
  }
}