#include <cstdio>
#include <iostream>
#include <vector>
#include <queue>
#include <utility>
#include <algorithm>
#include <functional>
#include <string>
#include <cstring>
#include <cmath>
#include <map>
#include <set>
#include <bitset>
#include <stack>
#include <iomanip>
#define NMAX 205
#define LMAX 5005
#define INF (1<<30)

using namespace std;

//FILE *fin = freopen("fisier.in", "r", stdin);
//FILE *fout = freopen("fisier.out", "w", stdout);

typedef pair<int, int> pii;

long long euclid(long long a, long long b) {
	int c;
	while (b) {
		c = a % b;
		a = b;
		b = c;
	}

	return a;
}

int main() {
	long long a, b, i, j, maxdist = -INF;;
	
	cin >> a >> b;

	for (i = 0; i < 1000; ++i) {
		for (j = 0; j < 1000; ++j) {
			if (b-j>0 && euclid(a + i, b - j) == 1) {
				maxdist = max(maxdist, b - j - (a + i) + 1);
			}
		}
	}

	cout << maxdist;

	return 0;
}