2016 ACM/ICPC Asia Regional Qingdao Online HDU5879

#include<stdio.h>
//#include<bits/stdc++.h>
#include<string.h>
#include<iostream>
#include<math.h>
#include<sstream>
#include<set>
#include<queue>
#include<map>
#include<vector>
#include<algorithm>
#include<limits.h>
#define MAXN (100000+10)
#define MAXM (100000)
#define inf 0x3fffffff
#define INF 0x3f3f3f3f
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define LL long long
#define ULL unsigned long long
using namespace std;
double a[1000000];
void cmd()
{
    a[0]=0;
    a[1]=1;
    for(long long i=2; i<1000000; i++)
    {
        a[i]=a[i-1]+1.0/(i*i);
    }
}
int main()
{
    string s;
    cmd();
    while(cin>>s)
    {
        if(s.length()>=7)
        {
            printf("%.5f\n",a[1000000-1]);
        }
        else
        {
            int b=0;
            for(int i=0; i<s.length(); i++)
            {
                b=b*10+s[i]-‘0‘;
            }
            printf("%.5f\n",a[b]);
        }
    }
    return 0;
}