nefu 117 素数个数的位数

题目：http://acm.nefu.edu.cn/test/problemshow.php?problem_id=117

思路：水

/*
 求10^n内素数的个数的位数
  = log(10)(num(n))+1
    num(n) = n/ln(n)
    num(10^n) = 10^n/(n*ln(10))
  = lg(10^n) - lg(n) - lg(ln(10)) + 1
  = n+1 -lg(n) - lb(ln(10))
*/
#include <cstdio>
#include <algorithm>
#include <cstring>
#include <cmath>
#include <iostream>
using namespace std;
int main()
{
    long long n;
    while(cin>>n)
    {
        cout<<int(n+1-log10(n)-log10(log(10)))<<endl;
    }
    return 0;
}