import java.util.*;

public class Main {
	static Scanner sc = new Scanner(System.in);
	public static void main(String[] args) {
		int n = sc.nextInt();
		int[] ar = new int[n];
		for (int i=0; i<n; i++) {
			int a = sc.nextInt();
			int b = sc.nextInt();
			ar[i] = b - a;
		}
		
		boolean exist = true;
		for (int i=0; i<n-1; i++) {
			if (ar[i]!=ar[i+1]) {exist = false;}
		}
		
		if (exist==false) {
			System.out.println(-1);
		}
		else {
			if (ar[0]<=0) {System.out.println(-1);}
			else {System.out.println(ar[0]);}
		}
	}
}