#include<bits/stdc++.h>
#define mp make_pair
#define PII pair<int,int>
#define fi first
#define se second
using namespace std;

long long a,b;

int Prime(long long x)
{
    int i,sq=sqrt(x);
    for (i=2;i<=sq;i++)
        if (x%i==0)
            return 1;
    return 0;
}

int main()
{
    long long i,j,l,sol=-1,dif;
    //freopen("date.in","r",stdin);
    //freopen("date.out","w",stdout);
    cin.sync_with_stdio(false);
    cin>>a>>b;
    for (i=a;Prime(i)==0 && __gcd(i,b)!=1 && i<=b;i++) ;
    if (i>b) i=b;
    if (__gcd(i,b)==1) sol=b-i+1;
    for (j=i-1;j>=a;j--)
        {
            dif=i-j;
            for (l=b-dif;l<=b;l++)
                if (__gcd(j,l)==1)
                    sol=max(sol,l-j+1);
        }

    cout<<sol<<"\n";
    return 0;
}