#include<stdio.h>

#include<string.h>

#include<algorithm>

#include<iostream>

#include<vector>

#include<queue>

#include<set>

using namespace std;

#define met(a, b) memset(a, b, sizeof(a))

#define N 1000005

#define INF 0x3f3f3f3f

typedef long long LL;

const LL MAX = 1e9;

double sum[N];

char str[N];

int main()

{

    for(int i=; i<N; i++)

        sum[i] = sum[i-] + 1.0/(1.0*i*i);

    ///for(int i=50000; i<N; i++)

     ///   printf("%d %.5f\n", N-1, sum[N-1]);

    while(scanf("%s", str) != EOF)

    {

        int len = strlen(str), num = ;

        if(len > )

        {

            printf("1.64493\n");

            continue;

        }

        for(int i=; i<len; i++)

            num = num* + str[i]-'';

        if(num >= N)

            printf("1.64493\n");

        else

            printf("%.5f\n", sum[num]);

    }

    return ;

}