//Code by Patcas Csaba

#include <vector>
#include <string> 
#include <set> 
#include <map> 
#include <queue> 
#include <bitset> 
#include <stack>
#include <list>

#include <numeric> 
#include <algorithm> 

#include <cstdio>
#include <fstream>
#include <iostream> 
#include <sstream> 
#include <iomanip>

#include <cctype>
#include <cmath> 
#include <ctime>
#include <cassert>

using namespace std;

#define LL long long
#define PII pair <int, int>
#define VB vector <bool>
#define VI vector <int>
#define VD vector <double>
#define VS vector <string>
#define VPII vector <pair <int, int> >
#define VVI vector < VI >
#define VVB vector < VB >

#define FORN(i, n) for(int i = 0; i < (n); ++i)
#define FOR(i, a, b) for(int i = (a); i <= (b); ++i)
#define FORD(i, a, b) for(int i = (a); i >= (b); --i)
#define REPEAT do{ 
#define UNTIL(x) }while(!(x)); 

#define SZ size()
#define BG begin() 
#define EN end() 
#define CL clear()
#define X first
#define Y second
#define RS resize
#define PB push_back
#define MP make_pair
#define ALL(x) x.begin(), x.end()

#define IN_FILE "a.in"
#define OUT_FILE "a.out"

unsigned long long a, b;

unsigned long long gcd(unsigned long long x, unsigned long long y)
{
  while (y != 0)
  {
    unsigned long long z = x % y;
    x = y;
    y = z;
  }
  return x;
}

int main()
{
	//Read data
	//freopen(IN_FILE, "r", stdin);
	//freopen(OUT_FILE, "w", stdout);
	

  cin >> a >> b;
	//Solve
  if (gcd(a, b) == 1) 
  {
    cout << b - a + 1 << endl;
    return 0;
  }

  while (a < b)
  {
    ++a;
    if (gcd(a, b) == 1) 
    {
      cout << b - a + 1 << endl;
      return 0;
    }
    --a;
    --b;
    if (gcd(a, b) == 1) 
    {
      cout << b - a + 1 << endl;
      return 0;
    }
    ++a;
  }
	

  cout << -1 << endl;
	return 0;
}