package yukicoder185;
import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Scanner sc=new Scanner(System.in);
		int n=sc.nextInt();
		int res=-1;
		for(int i=0;i<n;i++){
			int x=sc.nextInt();
			int y=sc.nextInt();
			if(i==0){
				if(y-x>0){
					res=y-x;
				}else{
					System.out.println(-1);
					return;
				}
			}else{
				if(y-x!=res){
					System.out.println(-1);
					return;
				}
			}
		}
		System.out.println(res);
	}
}