import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int N = sc.nextInt();
    int x0 = sc.nextInt();
    int y0 = sc.nextInt();
    int ans = y0 - x0;
    for(int i = 1; i < N; i++) {
      int x = sc.nextInt();
      int y = sc.nextInt();
      if(ans != y - x) {
        ans = -1;
        break;
      }
    }
    if(ans <= 0) ans = -1;
    System.out.println(ans);
  }
}