#include <cstdio>
#include <algorithm>

using namespace std;

inline bool gcd (long long a, long long b)
{
    while (b)
    {
        a %= b;
        swap (a, b);
    }

    return a == 1LL;
}

int main ()
{
    //freopen ("file.in", "r", stdin);

    int a, b;
    scanf ("%I64d %I64d", &a, &b);

    long long rez = -1;
    for (long long i = a; i <= min (a + 1000, b); ++i)
        for (long long j = max (a, b - 1000); j <= b; ++j)
            if (gcd (i, j)) rez = max (rez, j - i + 1);

    printf ("%I64d\n", rez);

    return 0;
}