CF 113C

 1 #include<iostream>
 2 #include<stdio.h>
 3 #include<cstring>
 4 #include<cstdlib>
 5 #include<bitset>
 6 using namespace std;
 7 const int maxn=3e8+10;
 8 
 9 bitset<maxn>prime;
10 int main()
11 {
12     int l,r,i,j;
13     while(scanf("%d%d",&l,&r)>0)
14     {
15         prime.set();
16         for(i=3;(j=i*i)<=r;i=i+2)//i*i<=r
17         {
18             if(prime[i]==false)continue;
19             for(;j<=r;j=j+i+i) // ！！！ j=j+i+i;
20                 prime[j]=false;
21         }
22         int ans=0;
23         if(l<=2 && 2<=r) ans++;
24         for(i=5;i<=r;i=i+4)
25             if(l<=i && prime[i]==true) ans++;
26         printf("%d\n",ans);
27     }
28     return 0;
29 }