B-1007

#include<cstdio>#include<string.h>int mark[100005], prime[100000], primesize;void unit(){    primesize = 0; memset(mark, 0, sizeof(mark));    for(int i = 2; i <= 100000; i++){        if(mark[i]==1) continue;        prime[primesize++] = i;        for(int j = 2*i; j <= 100000; j+=i) mark[j]=1;    }}int main(){    unit();    int n;    while(scanf("%d", &n)==1){        int cnt = 0;        for(int i = 1; prime[i]<=n && i<=primesize; i++)            if(prime[i]-prime[i-1]==2) cnt++;        printf("%d\n", cnt);    }    return 0;}