def monthly_savings(future_savings, years_to_retirement, interest_rate, compounds_per_year=2, startwith=0):
    """
    Calculates how much you need to save per month to retire given the parameters
    """
    r, m, n = interest_rate, compounds_per_year, years_to_retirement * 12
    efr = (1 + r/m) ** (m / 12) - 1
    already_secured = startwith * (1 + r / m) ** (m * years_to_retirement)
    future_savings -= already_secured
    return future_savings * efr / ((1 + efr) ** n - 1)