#include <iostream>
using namespace std;
long long a,b,i,j,best=-2;
long long gcd(long long a, long long b) { return b?gcd(b,a%b):a; }
int main() {
  cin>>a>>b;
  for (i=a; i<=b && i<=a+250; i++)
    for (j=b; j>=a && j>=b-250; j--)
      if (j-i>best && gcd(i,j)==1) best=j-i;
  cout<<best+1;
  return 0;
}