#include<iostream>
#include<math.h>
#include<algorithm>
#include<string.h>
using namespace::std;
int main()
{
	long int t,s,i=0,n,k=0;
	cin>>t;
	while(t>0)
	{
		cin>>n>>s;
		k=abs(s-n);
		if(n=='1' && s=='0')
		{
			cout<<"1";
		}
		else if(k>n)
		{
			cout<<"-1";
		}
		
	}
	
	
	return 0;
	
}