1 #include<iostream>
 2 #include<cstdio>
 3 #include<cmath>
 4 using namespace std;
 5 const int N=5000000;
 6 int vis[N];
 7 int main()
 8 {
 9     int l,r;
10     cin>>l>>r;
11     vis[0]=1;
12     vis[1]=1;
13     for(int i=2;i<=sqrt(r)+1;i++)
14      {
15          if(vis[i]==0)
16           {
17               for(int j=i*2;j<=r;j+=i)
18                {
19                    vis[j]=1;
20                 } 
21           }
22      }
23      int ans=0;
24      for(int i=l;i<=r;i++)
25       {
26           if(vis[i]==0)
27            {
28                ans++;
29            }
30       }
31       cout<<ans;
32 }