#include<stdio.h>
#include<algorithm>
using namespace std;

long long a,b;

long long gcd(long long a,long long b)
{
    if(b==0) return a;
    else
     return gcd(b,a%b);
}

void solve()
{
    if(a<=1) {printf("%I64d",b); return;}

    long long sol=-1;

    for(long long i=a;i<=a+5000 && i<b;i++)
     for(long long j=b;j>i && j>=b-10000;j--)
      if( gcd(i,j)==1 )
        sol=max(sol,j-i+1);

    printf("%I64d",sol);
}

int main()
{
    scanf("%lld %lld",&a,&b);

    solve();
    return 0;
}