1 #include<iostream>
 2 #include<cstdio>
 3 #include<cstdlib>
 4 #include<cstring>
 5 #include<ctime>
 6 #include<cmath>
 7 #include<algorithm>
 8 using namespace std;
 9 #define MAXN 5000010
10 #define FILE "read"
11 typedef long long ll;
12 ll n,x,cnt,len,pr[MAXN],prime[MAXN],isprime[MAXN];
13 ll gcd(ll a,ll b) {return !b?a:gcd(b,a%b);}
14 ll fast(ll a,ll b,ll mod) {ll sum=1;for(;b;b>>=1,a=a*a%mod)if(b&1)sum=sum*a%mod;return sum;}
15 inline ll read()
16 {
17     ll x=0,f=1;  char ch=getchar();
18     while(!isdigit(ch))  {if(ch=='-')  f=-1;  ch=getchar();}
19     while(isdigit(ch))  {x=x*10+ch-'0';  ch=getchar();}
20     return x*f;
21 }
22 void get()
23 {
24     for(ll i=2;i<=MAXN-10;i++)
25     {
26         if(!isprime[i])  prime[++cnt]=i;
27         for(ll j=1;j<=cnt&&prime[j]*i<=MAXN-10;j++)
28         {
29             isprime[prime[j]*i]=1;
30             if(i%prime[j]==0)  break;
31         }
32     }
33 }
34 void pre(ll p)
35 {
36     ll temp=p-1;
37     for(ll i=1;i<=cnt;i++)
38     {
39         if(temp%prime[i]==0)  pr[++len]=prime[i];
40         while(temp%prime[i]==0)  temp/=prime[i];
41     }
42     if(temp>1)  pr[++len]=temp;
43 }
44 bool check(ll d,ll p)
45 {
46     if(gcd(p,d)!=1)  return 0;
47     for(ll i=1;i<=len;i++)  if(fast(d,(p-1)/(pr[i]),p)==1)  return 0;
48     return 1;
49 }
50 int main()
51 {
52     freopen(FILE".in","r",stdin);
53     freopen(FILE".out","w",stdout);
54     n=read();  x=read();
55     get();
56     if(isprime[n+1])  {printf("-1\n");  return 0;}
57     pre(n+1);
58     for(ll i=x-1;i>1;i--)  if(check(i,n+1))  {printf("%I64d\n",i); return 0;}
59     printf("-1\n");
60     return 0;
61 }