#include<stdio.h>
 
int prime[200000]={},num[250000]={};
int main(void){
    int i,j,k=0,N,count;
    prime[k++]=2;
    prime[k++]=3;
 
    for(i=5;i<250000;i+=2){
        int flag=0;
        for(j=0;i>=prime[j]*prime[j];j++){
            if(i%prime[j]==0){
                flag=1;
                break;
            }
        }
        if(flag==0){prime[k++]=i;}
    }
 
     
     
    for(i=0;i<k;i++){
        num[prime[i]]=1;
    }
 
     
        while(1){
        scanf("%d",&N);
        if(N==0){break;}
        count=0;
        for(i=N+1;i<=2*N;i++){
            count+=num[i];
        }
 
        printf("%d\n",count);
 
    }
    return 0;
}