import java.util.Scanner;

public class Main{
	public static void main(String[] args){
		Scanner sc=new Scanner(System.in);
		while(sc.hasNext()){
			int n=sc.nextInt();
			int ans=sc.nextInt()-sc.nextInt(); ans=-ans;
			for(int i=1;i<n;i++){
				int x=sc.nextInt();
				int y=sc.nextInt();
				int temp=y-x;
				if(ans==-1) continue;
				if(ans!=temp) ans=-1;
			}
			System.out.println(ans<=0 ? -1 : ans);
		}
	}
}