#include <iostream>

using namespace std;

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

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;
}

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

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

    cout << answer << '\n';

    return 0;
}