import java.util.Arrays;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.Map;
import java.util.Scanner;
import java.util.Set;

import sun.security.util.ManifestDigester.Entry;

public class Main {
	
	public static void main(String[] args){
		Scanner sc = new Scanner(System.in);
		
		final int N = sc.nextInt();
		
		boolean first = true;
		boolean not_same = false;
		int same_num = -1;
		for(int i = 0; i < N; i++){
			final int X = sc.nextInt();
			final int Y = sc.nextInt();
			
			if(not_same){
				continue;
			}
			
			if(Y - X <= 0 || (!first && same_num != Y - X)){
				not_same = true;
				same_num = -1;
			}else if(first){
				first = false;
				same_num = Y - X;
			}
		}
		
		System.out.println(same_num);
	}
	
}