#include <iostream>

using namespace std;

long long a, b, aa, bb;

long long gcd(long long a, long long b){
    if(!b)
        return a;
    return gcd(b, a % b);
}

int main(){
    cin >> a >> b;
    aa = a;
    bb = b - 1;
    while(gcd(a, b) != 1 && a != b)
        ++a;
    if(a == b)
        a += 2;
    while(gcd(aa, bb) != 1 && aa != bb && bb - aa + 1 > b - a + 1)
        ++aa;
    if(aa == bb)
        aa += 2;
    if(bb - aa + 1 > b - a + 1)
        cout << bb - aa + 1;
    else
        cout << b - a + 1;
    return 0;
}