#include <bits/stdc++.h>
#define int64 long long
#define pb push_back
#define mp make_pair
#define INF numeric_limits<int>::max()
using namespace std;
int64 gcd(int64 a,int64 b)
{
    int64 r;
    r=a%b;
    while(r)
    {
        a=b;
        b=r;
        r=a%b;
    }
    return b;
}
int64 x,y;
int main() {
	cin>>x>>y;
	if(x==1 && y==1)
	{
		cout<<"1\n";
		return 0;
	}
	int64 mx=-1;
	for(int64 i=y;i>x;i--)
	{
		for(int64 j=x;j<i;j++)
		if(gcd(i,j)==1)
		{
			mx=max(mx,i-j+1);
			break;
		}
		if(mx>=i-x)break;
	}
	cout<<mx<<'\n';
	return 0;
}