#include<iostream>
#include<algorithm>
using namespace std;

long long gcd(long long a, long long b){
     while(a!=0 && b!=0)
       if(a>b) a %= b;
       else b %= a;
       
     return (a+b);        
}

long long i,j,a,b,sol;

int main(){
    cin>>a>>b;
    sol = -1;
    
    for(i=a; i<=a+5; i++)
      for(j=b; j>=b-5 && j>=i; j--)
        if(gcd(i, j)==1)
           sol = max(sol, j-i+1);
    
    cout<<sol;
    return 0;
}