#include<bits/stdc++.h>
using namespace std;
typedef long long ll;

void work(){
	ll a,b,c;
	cin>>a>>c;
	if(a==c){
		cout<<a<<'\n';
		return;
	}
	if(c>a){
		cout<<"-1\n";
		return;
	}
	b=a>>1;
	if(b==c){
		cout<<b<<'\n';
		return;
	}
	if(b<c*2){
		cout<<"-1\n";
		return;
	}
	ll m=b/c;
	if(b/m==c){
		cout<<b<<'\n';
		return;
	}
	cout<<"-1\n";
}

int main(){
	ios::sync_with_stdio(0);
	cin.tie(0);cout.tie(0);
	int T;  cin>>T;
	while(T--)work();
	return 0;
}

这个能过 #7~#10，赛时我加上特判前6个点的暴力就过了