/*
MindCoding 2016 Round 1
Vladimir VladimirM98 Milenkovic
*/
#include <bits/stdc++.h>
using namespace std;
const int N=200;
int done[N];
long long gcd(long long a,long long b)
{
    //printf("%lld %lld\n",a,b);
    if(b==0) return a;
    return gcd(b,a%b);
}
int main()
{
    long long x,y;
    scanf("%lld%lld",&x,&y);
    long long ans=-1ll;
    const int TRY=1234;
    for(long long b=y;b>=y-TRY;b--)
    for(long long a=x;a<=x+TRY;a++){
        if(a<=b&&gcd(a,b)==1ll) ans=max(ans,b-a+1);
    }
    printf("%lld\n",ans);
    return 0;
}