import java.util.ArrayList;
import java.util.HashSet;
import java.util.Scanner;
import java.util.TreeSet;

public class Main {
	static int N;
	static int[] X,Y;
	
	public static int solve(){
		int answer = Y[0] - X[0];
		
		if (answer < 0) return -1;
		
		for (int i = 1; i < N; i++) {
			if (Y[i] - X[i] != answer) return -1;
			else continue;
		}
		
		return answer;
	}
	
	public static void main(String[] args){
		Scanner S = new Scanner(System.in);
		
		N = S.nextInt();
		
		X = new int[N];
		Y = new int[N];

		for (int i = 0; i < N; i++) {
			X[i] = S.nextInt();
			Y[i] = S.nextInt();
		}
		
		System.out.println(solve());
	}
}