#include <iostream>

using namespace std;

bool prime(unsigned long long int  a,unsigned long long int  b)
{
    unsigned long long int s= a<b? a:b;
    for(unsigned long long int i=2;i<=s;i++)
        if(a%i==0&&b%i==0)
        return false;
    return true;
}

int main()
{
    unsigned long long int a,b;
    cin>>a>>b;

    while(a<=b)
    {
        while(!prime(a--,b--));
        cout<<b-a+1;
        return 0;
    }
    cout<<-1;
    return 0;
}