import java.util.*;

public class Exercise96{
  public static void main (String[] args){

    Scanner sc = new Scanner(System.in);

    int n = sc.nextInt();

    int[][] x = new int[n][2];
    for(int i = 0; i < n; i++){
      x[i][0] = sc.nextInt();
      x[i][1] = sc.nextInt();
    }

    int a = x[0][1] - x[0][0];
    if(a < 1){
      System.out.println(-1);
      return;
    }

    for(int i = 1; i < n; i++){
      if(x[i][1] - x[i][0] != a){
        System.out.println(-1);
        return;
      }
    }
    System.out.println(a);
  }
}