import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] x = new int[n];
		int[] y = new int[n];
		for(int i = 0 ; i < n ; i++) {
			x[i] =sc.nextInt();
			y[i] = sc.nextInt();
		}
		long[] a = new long[n];
		for(int i = 0 ; i < n ; i++) {
			a[i] = y[i] - x[i];
		}
		for(int i = 0 ; i < n ; i++) {
			for(int j = i + 1 ; j < n ; j++) {
				if(a[i] != a[j] || a[i] <= 0) {
					System.out.println(-1);
					return;
				}
			}
		}
		System.out.println(a[0]);
 	}
}