//Dragan Andrei Gabriel
//University of Bucharest

#include <cstdio>
#include <cassert>
#include <cmath>
#include <cstring>
#include <ctime>
#include <cstdlib>
#include <iostream>
#include <iomanip>
#include <sstream>
#include <istream>
#include <ostream>
#include <deque>
#include <map>
#include <queue>
#include <stack>
#include <set>
#include <vector>
#include <algorithm>
#include <functional>
#include <string>
#include <utility>
using namespace std;

long long cmmdc(long long x, long long y)
{
    long long r = x % y;
    while (r != 0)
    {
        x = y;
        y = r;
        r = x % y;
    }
    return y;
}

long long x, y;
int main()
{
    #ifndef ONLINE_JUDGE
    freopen("a.in", "r", stdin);
    freopen("a.out", "w", stdout);
    #endif

    scanf("%lld %lld", &x, &y);

    if (cmmdc(x, y) == 1)
    {
        printf("%lld\n", y - x + 1);
        return 0;
    }

    long long rez = -1;
    long long st = 1;
    long long dr = y - x + 1;


    while (st <= dr)
    {
        long long mij = (st + dr) / 2;
        bool ok = false;
        for (int i = x; i <= y - mij; i++)
            if (cmmdc(i, i + mij) == 1)
            {
                ok = true;
                break;
            }
        if (ok == true)
        {
            rez = mij;
            st = mij + 1;
        }else
            dr = mij - 1;
    }

    printf("%lld\n", rez);
    return 0;
}