#include <iostream>
#include <fstream>
#include <bitset>
#include <vector>
using namespace std;
#define llu long long int
const int zecela8 = 100000003;
bitset< zecela8 > v(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 && a%2 == 0)
        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);
    while(b>a)
    {
        bool ok = false;
        for(int prime:primesofa)
            if(b%prime == 0)
            {
                ok = true;
                break;
            }
        if(ok == false)
            break;
        else
            b--;
    }
    if(b == a)
        cout<<"-1";
    else
        cout<<b-a+1;
    return 0;
}