import java.util.Scanner;

class No_185 {
	public static void main(String args[]) {
		Scanner stdIn = new Scanner(System.in);		
		int num = stdIn.nextInt();
		int [] unknown = new int [num];

		for (int i = 0; i < num; i++) {
			int x1 = stdIn.nextInt();
			int x2 = stdIn.nextInt();
			if ( x1 < x2 ) {
				unknown[i] = x2 - x1;
			} else {
				System.out.println(-1);
				System.exit(0);
			}
		}

		for (int i = 0; i < num-1; i++) {
			if (unknown[i] != unknown[i+1]) {
				System.out.println(-1);
				System.exit(0);
			}
		}
		System.out.println(unknown[0]);
	}
}