#include <iostream>
#include <cmath>  
#include <cassert>  
#include <vector>
#include <iomanip>
using namespace std;

double NormalSeries(size_t N)
{
    double sum = 0.0;
    for (size_t i = 1; i <= N; ++i)
    {
        sum += 1.0 / (double(i) * double(i));
    }
    return sum;
}

double KahanSeries(size_t N)
{
    double sum = 0.0;
    double c = 0.0; 
    for (size_t i = 1; i <= N; ++i) {
        double term = 1.0 / (double(i) * double(i));
        double y = term - c;
        double t = sum + y;
        c = (t - sum) - y;
        sum = t;
    }
    return sum;
}

int main()
{
    const double pi2_over_6 = (M_PI * M_PI) / 6.0;
    // vector<size_t> ns = {10, 100, 1000, 10000, 100000, 1000000, 10000000, 100000000};
    vector<size_t> ns = {1000000, 10000000, 100000000, 500000000, 1000000000};


    for (size_t n : ns)
    {
        double s_norm  = NormalSeries(n);
        double s_kahan = KahanSeries(n);
        double err_norm = fabs(s_norm - pi2_over_6);
        double err_kahan = fabs(s_kahan - pi2_over_6);
        
        cout.setf(ios::fixed);
        cout.precision(10);
        cout << "\n--- n = " << n << " ---" << endl;
        cout << "Normal summation: " << s_norm << endl;
        cout << "Normal error: " << err_norm << endl;
        cout << "Kahan summation:  " << s_kahan  << endl;
        cout << "Kahan error:  " << err_kahan  << endl;
    }

    return 0;
}