#include<cstdlib>
#include<vector>
#include<queue>
#include<map>
#include<cctype>
#include<string>
#include<cstring>
#include<algorithm>
using namespace std;

#define fisier 0

#if fisier
 #include<fstream>
 ifstream cin("fisier.in");
 ofstream cout("fisier.out");
#else
 #include<iostream> 
#endif

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;
    
    #if fisier
     cin.close();
     cout.close();
    #endif
    return 0;
}