#include <stdio.h>

using namespace std;
long long n,m,i,j,dif;
long long shl,tmp;
bool eu,ev;
long long gcd(long long u, long long v)
{
    // simple cases (termination)
    if (u == v)
        return u;

    if (u == 0)
        return v;

    if (v == 0)
        return u;

    // look for factors of 2
    if (~u & 1) // u is even
    {
        if (v & 1) // v is odd
            return gcd(u >> 1, v);
        else // both u and v are even
            return gcd(u >> 1, v >> 1) << 1;
    }

    if (~v & 1) // u is odd, v is even
        return gcd(u, v >> 1);

    // reduce larger argument
    if (u > v)
        return gcd((u - v) >> 1, v);

    return gcd((v - u) >> 1, u);
}
int main()
{
    scanf ("%lld%lld",&n,&m);
    dif=-1;
    for (i=n;i<=n+1000&&i<m;i++)
        for (j=m;j>=m-1000&&j>n&&j>i;j--)
            if (gcd(i,j)==1)
                if (j-i+1>dif)
                    dif=j-i+1;
    printf ("%lld",dif);
    return 0;
}