import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n = scanner.nextInt();
        int[] a = new int[n];
        int[] b = new int[n];
        
        for (int i = 0; i < n; i++) {
            a[i] = scanner.nextInt();
            b[i] = scanner.nextInt();
        }
        
        int result = -1;
        List<Integer> vec = new ArrayList<>();
        
        for (int i = 0; i < n; i++) {
            vec.add(b[i] - a[i]);
        }
        
        Collections.sort(vec);
        
        int first = vec.get(0);
        int last = vec.get(vec.size() - 1);
        if (first == last && last > 0) {
            result = last;
        }
        
        System.out.println(result);
    }
}