import java.util.*;
import java.math.*;
public class Main{
	public static void main(String... args){
		Scanner scan = new Scanner(System.in);
		int n = scan.nextInt();
		System.out.println(Calc(n,scan));
	}

	public static int Calc(int n,Scanner scan){
		int result = -1;
		for(int i = 0; i < n; i++){
			int x = scan.nextInt();
			int y = scan.nextInt();

			int fx = y-x;

			if(i == 0 && 0 < fx){
				result = fx;
			}else if(result != fx){
				result = -1;
			}else if(fx <= 0){
				result = -1;
			}
		}

		return result;
	}
}