import java.util.HashSet;
import java.util.Scanner;


public class Main {
	static int N;
	static int[] X, Y;
			
	static private void solve()
	{
		HashSet<Integer> hs = new HashSet<Integer>();
		
		for (int i = 0; i < N; i++) {
			hs.add(new Integer(Y[i] - X[i]));
		}
		
		if (hs.size() > 1) {
			System.out.println("-1");
			return;
		}
		
		for (Integer i : hs) {
			if(i.intValue() <= 0)
			{
				System.out.println("-1");
				return;
			}
			
			System.out.println(i);
		}
	}
	
	static public void main(String[] args)
	{
		Scanner sca = new Scanner(System.in);
		
		N = sca.nextInt();
		
		X = new int[N];
		Y = new int[N];
		for (int i = 0; i < N; i++) {
			X[i] = sca.nextInt();
			Y[i] = sca.nextInt();
		}
		
		solve();
	}
}