#include <cstdio>
#include <iostream>
#include <algorithm>
#include <cmath>
using namespace std;
long long a,b;
long long ans = -1;
long long i;
long long gcd(long long a,long long b)
{
    long long r;

    while (b!=0)
    {
        r = a%b;
        a = b;
        b = r;
    }
    return a;
}

int main()
{
    cin>>a>>b;

    long long lim = max(b-400,a);
    for (i = b; i >= lim;i--)
    {
        long long nr = a;

        while (gcd(nr,i) != 1 && nr < i)
            nr++;
        if (nr < i)
        {
            ans = max(ans, i-nr+1);
        }
    }
    cout<<ans;

    return 0;
}