传送门
筛法素数打表->枚举
╮(╯-╰)╭
#include <stdio.h>
#include <math.h>
#include <string.h>
#define INF 1<<20
int main()
{
long long i,j;
char isPrime[INF];
memset(isPrime,1,sizeof(isPrime));
isPrime[0] = isPrime[1] = 0;
for(i=2; i<=INF; i++)
for(j=i*2; j<=INF; j+=i)
isPrime[j] = 0;
int k,N,ans = 0;
scanf("%d",&N);
for(k=3; k<=N; k+=2)
if(isPrime[k])
if(isPrime[k]-isPrime[k-2]==0)
ans++;
printf("%d",ans);
return 0;
}