#include <iostream>

using namespace std;

long long A,B;
long long x,y;
long long answer;

long long cont = 0;

bool prime(long long a)
{
    if (a == 1)
        return true;

    for (int i = 2; i*i <= a; ++i)
    {
        if (a % i == 0)
        {
            return false;
        }
    }

    return true;
}

long long gcd(long long a, long long b) {
    return b == 0 ? a : gcd(b, a % b);
}

int main()
{
    cin >> A >> B;
    x = A;
    y = B;

    while (x <= y)
    {
        cont++;

        if (cont == 5000)
            break;

        if (prime(x))
        {
            answer = max(answer, B - x + 1);
            break;
        }
        if (prime(y))
        {
            answer = max(answer, y - A + 1);
            break;
        }
        x++;
        y--;
    }

    for (long long i = A; i <= min(A+2000,B-1); ++i)
    {
        for (long long j = B; j >= max(B-2000,A+1); --j)
        {
            if (gcd(i,j) == 1)
            {
                answer = max(answer,j-i+1);
            }
        }
    }

    cout << answer << '\n';

    return 0;
}