#include <bits/stdc++.h>
#define pb push_back
#define mp make_pair
#define minim(x,y) (x<y?x:y)
#define maxim(x,y) (x>y?x:y)

const int Nmax=100005;
const int N=1005;

using namespace std;

long long x,y,i,j,a,b,Max;

inline bool prim(long long x)
{
    long long i;
    if(x==2LL) return 1;
    if(x%2LL==0LL) return 0;

    for(i=3;i*i<=x;i+=2LL)
    if(x%i==0LL) return 0;
    return 1;
}
inline long long gcd(long long a,long long b)
{
    long long r=a%b;
    while(r!=0LL) a=b, b=r, r=a%b;
    return b;
}
int main()
{
     #ifndef ONLINE_JUDGE
     freopen("a.in","r",stdin);
     freopen("a.out","w",stdout);
     #endif

     scanf("%lld%lld",&a,&b);
     x=a;y=b;
     while(!prim(x) && x<b) ++x;
     while(!prim(y) && y>a) --y;

     Max=-1LL;
     for(i=a;i<=x;++i)
     for(j=b;j>=y;--j)
     if(j-i>Max)
     {
        if(gcd(i,j)==1) Max=j-i;
     }
     printf("%lld\n",Max+1LL);

     return 0;
}