#include <iostream>
#include <fstream>
#include <bitset>
#include <vector>
using namespace std;
#define llu long long int
const int zecela8 = 100000003;
const long long  zecela15 = 1000000000000003;
bitset< zecela8 > v(0);
bitset< zecela15 >z(0);
vector< llu > primesofa;
llu a,b;
int main()
{

    for(int i = 3 ; i*i <= zecela8 ; i ++)
        if(v[i] == 0)
            for(int j = i*i ; j <= zecela8 ; j += 2*i )
                v[j] = 1;
    cin >> a >> b;
    if( 2 <=a )
        primesofa.push_back(2);
    for(int i = 3 ; i <= zecela8 ; i+= 2)
        if (v[i] == 0 && i <= a && a%i == 0)
            primesofa.push_back(i);
    for(int prime:primesofa)
        if(z[prime] == 0)
            for(int j = prime*prime ; j<= zecela15 ; j+= 2*prime)
                z[j] = 1;
    while(b>a && z[b] == 1)
    {
        b--;
        cout<<"z["<<b<<"]=="<<z[b]<<"\n";
    }
    if(b == a)
        cout<<"-1";
    else
        cout<<b-a+1;
    return 0;
}