#include <bits/stdc++.h>
#define FOREACH(it,c) for( __typeof((c).begin()) it=(c).begin();it!=(c).end();it++)
#define FOR(a,b,c) for(int a=(b);a<=(c);++a)
#define ROF(a,b,c) for(int a=(b);a>=(c);--a)
#define dbg(x) cout<<#x<<" = "<<(x)<<"\n";
#define pii pair<int,int>
#define pll pair< ll, ll >
#define pull pair< ull, ull >
#define mp make_pair
#define pb push_back
#define fi first
#define se second
#define ll long long
#define ull unsigned long long
#define tata NULL
using namespace std;
const int NMAX = 1000004;
long long a, b;
int MAX = 20000000;
int MAX2 = 2000;
int main()
{
    #ifndef ONLINE_JUDGE
        freopen("data.in","r",stdin);
    #endif // ONLINE_JUDGE
    cin.sync_with_stdio(false);
    cin.tie(tata);
    cin >> a >> b;
    long long sol = -1;
    int step = 0;
    for(long long i=a;i <= b && ++step <= MAX; ++i)
    {
        int step2 = 0;
        long long j = b;
        while(j >= a && ++step2<=MAX2)
        {
            if(__gcd(i,j)==1)
            {
                sol = max(j-i+1,sol);
                break;
            }
            --j;
        }
    }
    swap(MAX,MAX2);
    for(long long i=a;i <= b && ++step <= MAX; ++i)
    {
        int step2 = 0;
        long long j = b;
        while(j >= a && ++step2<=MAX2)
        {
            if(__gcd(i,j)==1)
            {
                sol = max(j-i+1,sol);
                break;
            }
            --j;
        }
    }
    cout<<sol<<"\n";
    return 0;
}