#include<iostream>
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 a,b,sol;

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

    while(a<=b && gcd(a, b)!=1) ++a;
    
    if(a<=b && gcd(a, b)==1) sol = b-a+1;
    else sol = -1;
    
    cout<<sol;
    
    return 0;
}