#include <iostream>

using namespace std;

long long a,b,Max,i,st,dr;
int ok;

long long cmmdc(long long a,long long b)
{
    long long r=0;
      while(a > 0)
        {
            r = b % a;
            b = a;
            a = r;
        }
    return b;
}


int main()
{
    cin>>a>>b;
    Max=0;
    for(i=a;i<=(a+b)/2;++i)
    {
        if(b-i+1 < Max) break;
        st=i;
        dr=b;
        ok=0;
        while(st < dr)
        {
            if(st % 2 == 0 && dr%2 == 0) --dr;
            if(cmmdc(st,dr) != 1) --dr;
                else break;
        }
        Max=max(dr-st+1,Max);
    }
    cout<<Max;
    return 0;
}