import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int N = scan.nextInt();
		int []X = new int[N];
		int []Y = new int[N];
		for(int i = 0; i < N; i++) {
			X[i] = scan.nextInt();
			Y[i] = scan.nextInt();
		}
		scan.close();
		int z = Y[0] - X[0];
		if(z <= 0) {
			System.out.println(-1);
			System.exit(0);
		}
		for(int i = 1; i < N; i++) {
			int temp = Y[i] - X[i];
			if(z != temp) {
				System.out.println(-1);
				System.exit(0);
			}
		}
		System.out.println(z);
	}
}