C :

#include <stdio.h>
#include <stdlib.h>

/* run this program using the console pauser or add your own getch, system("pause") or input loop */
int f(a)
{
    int i,y=0;
    for(i=a;i<a*2;i++)
    {
    	y=y+i;
    }
    return y;
}
int main(int argc, char *argv[]) {
	int n,i;
	double s=0;
	scanf("%d",&n);
	for(i=1;i<n+1;i++)
	{
		s=s+1.0/f(i);
	}
	printf("s=%f",s);
	return 0;
}

C++ :

#include <iostream>
#include <iomanip>
using namespace std;

double fun(int x);

int main()
{
double sum;int n;
cin>>n;
for(int i=1;i<=n;i++)
{
sum+=fun(i);
}
cout<<"s="<<fixed<<setprecision(6)<<sum;
}

double fun(int x)
{
int y=(x+2*x-1)*x/2;
double z=1/(double)y;
return z;
}

C# :

using System;

namespace ACMclubzjicm
{
    class Test_1337
    {
        static void Main()
        {
            string stIn = Console.ReadLine().Trim();
            int n = int.Parse(stIn);
            double s = 0;
            f(n, ref s);
            Console.Write("s={0:F6}", s);
            //Console.ReadLine();
            return;
        }

        private static void f(int n, ref double s)
        {
            s += 2 / (3 * Math.Pow(n, 2) - n);
            if (n > 1)
            {
                n--;
                f(n, ref s);
            }
            return;
        }
    }
}