#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<ctime>
bool  a[5100000];
int  n,m,s=0;
int  main()
{
    scanf("%d%d",&n,&m);
    if(n>m)
    {
        int  t=n;n=m;m=t;
    }
    if(n<=2)
    {
        s++;
        n=3;
    }
    if(n%2==0)n++;
    memset(a,True,sizeof(a));
    for(register  int  i=3;i<=sqrt(m);i+=2)
    {
        if(a[i/2]==True)
        {
            for(register  int  j=i*3;j<=m;j+=i*2)a[j/2]=False;
        }
    }
    for(register  int  i=n;i<=m;i+=2)
    {
        if(a[i/2]==True)s++;
    }
    printf("%d\n",s);
    return 0;
}

#include<cstdio>
#include<cstring>
#include<cstdlib>
#include<cmath>
#include<ctime>
bool  a[5100000];
int  n,m,s=0;
int  main()
{
    scanf("%d%d",&n,&m);
    if(n>m)
    {
        int  t=n;n=m;m=t;
    }
    if(n<=2)
    {
        s++;
        n=3;
    }
    if(n%2==0)n++;
    memset(a,True,sizeof(a));
    for(register  int  i=3;i<=sqrt(m);i+=2)
    {
        if(a[i/2]==True)
        {
            for(register  int  j=i*3;j<=m;j+=i*2)a[j/2]=False;
        }
    }
    for(register  int  i=n;i<=m;i+=2)
    {
        if(a[i/2]==True)s++;
    }
    printf("%d\n",s);
    return 0;
}