import java.util.Scanner;

public class No0185 {
	public static void main(String[] args) {
		try (Scanner sc = new Scanner(System.in)) {
			int N = sc.nextInt();
			int ans = 0;

			for (int i = 0; i < N; i++) {
				
				if (i == 0) {
					int x = sc.nextInt();
					int y = sc.nextInt();
					
					if (y - x < 1) {
						ans = -1;
					}
				}
				else {
					int x = sc.nextInt();
					int y = sc.nextInt();
					
					if (y - x != ans) {
						ans = -1;
						break;
					}
				}
				
			}
			System.out.println(ans);

		}
	}
}