import java.util.Scanner;


public class Main_yukicoder185 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();
        int prev = 0;
        boolean flag = true;
        for (int i = 0; i < n; i++) {
        	int x = sc.nextInt();
        	int y = sc.nextInt();
        	if (i > 0) {
        		if (y - x != prev) {
        			flag = false;
        			break;
        		}
        	} else {
        		prev = y - x;
        	}
        }

        if (flag && prev > 0) {
        	System.out.println(prev);
        } else {
        	System.out.println("-1");
        }
        
        sc.close();
    }
}