package yukicoder;

import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) {
		BufferedReader read = new BufferedReader(new InputStreamReader(System.in));
		try {

			int N = Integer.parseInt(read.readLine());
			int num;
			String[] box = read.readLine().split(" ");

			int x, y;
			x = Integer.parseInt(box[0]);
			y = Integer.parseInt(box[1]);

			num = y - x;
			if (num < 0) {
				System.out.println(-1);
				return;
			}
			int i;
			for (i = 1; i < N; ++i) {
				box = read.readLine().split(" ");

				x = Integer.parseInt(box[0]);
				y = Integer.parseInt(box[1]);

				if (num != (y - x)) {
					break;
				}
			}

			if (i < N) {
				System.out.println(-1);
			} else {
				System.out.println(num);
			}

		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}