#include<bits/stdc++.h>
//#include<atcoder/all>
using namespace std;
//using namespace atcoder;
typedef int64_t ll;
//using mint=modint998244353;

typedef long double ld;
const ll MOD=1000000007;
const ll MODA=998244353;
ll vx[4]={0,1,0,-1};
ll vy[4]={1,0,-1,0};
#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
long long gcd(long long a,long long b){
    a=abs(a);
    b=abs(b);
    ll gcdmax=max(a,b);
    ll gcdmin=min(a,b);
    if(gcdmin==0)return gcdmax;
    while(true){
        if(gcdmax%gcdmin==0)break;
        else gcdmax%=gcdmin;
        swap(gcdmin,gcdmax);
    }
    return gcdmin;
}
ll pow(ll N,ll P,ll M){
    if(P==0)return 1;
    else if(P%2==0){
        ll t=pow(N,P/2,M);
        return t*t%M;
    }
    else return N*pow(N,P-1,M)%M;
}
ll pow(ll N,ll P){
    if(P==0)return 1;
    else if(P%2==0){
        ll t=pow(N,P/2);
        return t*t;
    }
    else return N*pow(N,P-1);
}

vector<ll> fac;
vector<ll> finv;
vector<ll> inv;
void COMinit(ll N,ll P){
    rep(i,N+1){
        if(i==0){
            fac.push_back(1);
            finv.push_back(1);
            inv.push_back(1);
        }
        else if(i==1){
            fac.push_back(1);
            finv.push_back(1);
            inv.push_back(1);
        }
        else{
            fac.push_back(fac.at(i-1)*i%P);
            inv.push_back(P-inv.at(P%i)*(P/i)%P);
            finv.push_back(finv.at(i-1)*inv.at(i)%P);
        }
    }
}

ll COM(ll n,ll k,ll P){
    if(n<k)return 0;
    if(n<0||k<0)return 0;
    return fac.at(n)*(finv.at(k)*finv.at(n-k)%P)%P;
}

vector<ll> find_divisor(ll N){
    vector<ll> A;
    for(ll i=1;i*i<=N;i++){
        if(N%i==0)A.push_back(i);
    }
     ll l=A.size();
    rep(i,l){
        if(A.at(l-i-1)*A.at(l-i-1)!=N)A.push_back(N/A.at(l-1-i));
    }
    return A;
}

int main(){
    ll T;
    cin>>T;
    vector<ll> Ans(T);
    rep(t,T){
        ll a,b;
        cin>>a>>b;
        if(b==a){
            Ans[t]=0;
            continue;
        }
        ll k=b-a;
        vector<ll> d=find_divisor(k);
        Ans[t]=-1;
        for(ll x:d){
            if(x>=a){
                Ans[t]=x-a;
                break;
            }
        }
    }
    for(ll x:Ans)cout<<x<<endl;
}