import java.util.Scanner;

public class No185 {
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		int[] input = new int[N*2];
		for (int i=0; i< N * 2; i++)
			input[i] = sc.nextInt();
		int dif = input[1] - input[0];
		int count = 0;
		
		if(dif <= 0)
			System.out.println("-1");
		else{
			for(int i=0; i < N * 2; i = i + 2){
			if(input[i] + dif == input[i+1])
				count++;
			}
		if(count ==N )
			System.out.println(dif);
		else
			System.out.println("-1");
	    
		sc.close();
		}
	}
}