#include <bits/stdc++.h>

using namespace std;

# define pb push_back
# define mp make_pair
# define FORN( a , b , c ) for ( int a = b ; a <= c ; ++ a )
# define FORNBACK( a , b , c ) for ( int a = b ; a >= c ; -- a )

int main()
{
    ios :: sync_with_stdio ( false ) ;

   // freopen( "input" , "r" , stdin ) ;
   // freopen( "output" , "w" , stdout ) ;
    long long a , b ;
    cin >> a >> b ;
    while ( b > a and __gcd ( a , b ) != 1 ) -- b ;
    if ( b == a ) cout << -1 ;
    else cout << b - a + 1 ;
    //assert ( largest == b - a + 1 ) ;
    return 0 ;
}