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

bool comp(int a, int b){
    return a>b;
}

ll primo[1000000];
ll vetor[1000000];
ll aux[1000];
int main(){
    for(ll i = 1;i<=1000000;i++){
            primo[i]=1;
        }
        for(ll i = 2;i<=1000000;i++){
            if(primo[i]){
                    for(ll j = 2*i;j<=1000000;j+=i){
                        primo[j] = 0;
                    }
            }
    }
    int a, b, c, d, z = 0, maior = 0;
    bool p = false;
    cin>>a>>b;
    for(int i = a;i<=b;i++){
        if(primo[i]==1){
            aux[z] = i;
            z++;
        }
    }

    for(int i = 1;i<=z;i++){
        int t = aux[i]-aux[i-1];
        vetor[aux[i]-aux[i-1]]++;
    }

    int saida = 0, q = 0;
    for(int i = 0;i<=b;i++){
        if(vetor[i]>saida){
            saida = vetor[i];
            q = i;
        }
    }

    if(q==0)cout<<"-1"<<endl;
    else cout<<q<<endl;
}