#include <cstdlib>
	#include <cmath>
	#include <climits>
	#include <cfloat>
	#include <map>
	#include <set>
	#include <iostream>
	#include <string>
	#include <vector>
	#include <algorithm>
	#include <sstream>
	#include <complex>
	#include <stack>
	#include <queue>
	#include <cstdio>
	#include <cstring>
	#include <iterator>
	#include <bitset>
	#include <unordered_set>
	#include <unordered_map>
	//#include <utility>
	//#include <memory>
	//#include <functional>
	//#include <deque>
	//#include <cctype>
	//#include <ctime>
	//#include <numeric>
	//#include <list>
	//#include <iomanip>
	
	//#if __cplusplus >= 201103L
	//#include <array>
	//#include <tuple>
	//#include <initializer_list>
	//#include <forward_list>
	//
	//#define cauto const auto&
	//#else
	
	//#endif
	
	using namespace std;
	
	
	typedef long long ll;
	typedef pair<int,int> pii;
	typedef pair<ll,ll> pll;
	
	typedef vector<int> vint;
	typedef vector<vector<int> > vvint;
	typedef vector<long long> vll, vLL;
	typedef vector<vector<long long> > vvll, vvLL;
	
	#define VV(T) vector<vector< T > >
	
	template <class T>
	void initvv(vector<vector<T> > &v, int a, int b, const T &t = T()){
	    v.assign(a, vector<T>(b, t));
	}
	
	template <class F, class T>
	void convert(const F &f, T &t){
	    stringstream ss;
	    ss << f;
	    ss >> t;
	}
	
	
	#define reep(i,a,b) for(int i=(a);i<(b);++i)
	#define rep(i,n) reep((i),0,(n))
	#define ALL(v) (v).begin(),(v).end()
	#define PB push_back
	#define F first
	#define S second
	#define mkp make_pair
	#define RALL(v) (v).rbegin(),(v).rend()
	// #define DEBUG
	#ifdef DEBUG
	#define dump(x)  cout << #x << " = " << (x) << endl;
	#define debug(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << endl;
	#else
	#define dump(x) 
	#define debug(x) 
	#endif
	
	#define MOD 1000000007LL
	#define EPS 1e-8
	static const int INF=1<<24;
	
	void mainmain(){
		int n;
		cin>>n;
		rep(i,n){
			int a,b;
			cin>>a>>b;
			if(b-a<=0) cout<<-1<<endl;
			else cout<<b-a<<endl;
		}
	}
	
	
	signed main() {
	    mainmain();
	}