Hjälp! Hur håller man j**** många decimaler i VB?

#include <functional>
#include <iostream>
#include <sstream>
#include <gmpxx.h>

using namespace std;

class Matrix {
    mpz_class m[4];
public:
    Matrix(const mpz_class &q, const mpz_class &r,
           const mpz_class &s, const mpz_class &t) {
        m[0] = q; m[1] = r;
        m[2] = s; m[3] = t;
    }

    const mpz_class &operator()(int row, int col) const {
        return m[(row<<1) + col];
    }
};

Matrix operator*(const Matrix& lhs, const Matrix& rhs) {
    return Matrix(lhs(0,0) * rhs(0,0) + lhs(0,1) * rhs(1,0),
                  lhs(0,0) * rhs(0,1) + lhs(0,1) * rhs(1,1),
                  lhs(1,0) * rhs(0,0) + lhs(1,1) * rhs(1,0),
                  lhs(1,0) * rhs(0,1) + lhs(1,1) * rhs(1,1));
}

Matrix lfts(int k)
{
    return Matrix(k, 4*k+2, 0, 2*k+1);
}

int extr(const Matrix &z, int x)
{
    mpz_class bn = (z(0,0) * x + z(0,1)) / (z(1,0) * x + z(1,1));
    return bn.get_si();
}

bool is_safe(const Matrix &z, int n)
{
    return n == extr(z, 4);
}

int next(const Matrix &z)
{
    return extr(z, 3);
}

Matrix prod(const Matrix &z, int n)
{
    return Matrix(10, -10 * n, 0,  1) * z;
}

void pi(function<void(int,int)> digit_fn, int num_digits)
{
    Matrix z(1, 0, 0, 1);
    int k = 1;
    int n = 0;

    while (n < num_digits) {
        int y = next(z);

        if (is_safe(z, y)) {
            digit_fn(y, n);
            z = prod(z, y);
            ++n;
        } else {
            z = z * lfts(k);
            ++k;
        }
    }
}

void show_digit(int d, int n)
{
    cout << d;
    if (n == 0)
        cout << '.';
    cout.flush();
}

int main(int argc, char *argv[])
{
    int N = 10;

    if (argc > 1)
        stringstream(argv[1]) >> N;

    pi(show_digit, N);
    cout << endl;
}