#include<bits/stdc++.h>
using namespace std;
#define FOR(i,a,b) for(int i = (a); i <= (b); ++i)
#define FORD(i,a,b) for(int i = (a); i >= (b); --i)
#define RI(i,n) FOR(i,1,(n))
#define REP(i,n) FOR(i,0,(n)-1)
#define mini(a,b) a=min(a,b)
#define maxi(a,b) a=max(a,b)
#define mp make_pair
#define pb push_back
#define st first
#define nd second
#define sz(w) (int) w.size()
typedef vector<int> vi;
typedef long long ll;
typedef long double ld;
typedef pair<int,int> pii;
const int inf = 1e9 + 5;
const int nax = 1e6 + 5;



int main() {
	ll a, b;
	scanf("%lld%lld", &a, &b);
	ll ans = -1;
	REP(i, 1000) REP(j, 1000) {
		ll aa = a + i;
		ll bb = b - j;
		if(aa <= bb && a <= aa && aa <= b && a <= bb && bb <= b)
			if(__gcd(aa, bb) == 1)
				maxi(ans, bb - aa + 1);
	}
	printf("%lld\n", ans);
	return 0;
}