#include <cstdio>
#include <cstring>

const int MAXN = 1e6+5;
double sum[MAXN];
char s[MAXN];

void init()
{
    sum[0] = 0;
    for(int i = 1; i <= 1000000; ++i){
        sum[i] = sum[i-1]+1.0/(i*1.0*i);
    }
}

int main()
{
    init();
    while(~scanf("%s", s)){
        int len = strlen(s);
        if(len >= 7){
            printf("1.64493\n");
            continue;
        }
        int n = 0;
        for(int i = 0; i < len; ++i)
            n = n*10+s[i]-‘0‘;
        printf("%.5f\n", sum[n]);
    }
    return 0;
}