help debug in C#

// Program decides tuition based on several criteria:

// 1 - 12 credit hours @ $150 per credit hour

// 13 - 18 credit hours, flat fee $1900

// over 18 hours, $1900 plus $100 per credit hour over 18

// If year in school is 4, there is a 15% discount

using System;

using static System.Console;

class DebugFour3

{

static void Main()

{

int credits, year;

string inputString;

double tuition;

const int LOWCREDITS = 12;

const int HIGHCREDITS = 18;

const double HOURFEE = 15000;

const double DISCOUNT = 0.15;

const double FLAT = 1900.00;

const double RATE = 100.00;

const int SENIORYEAR = 4;

WriteLine("How many credits? ");

inputString = ReadLine();

credits = Convert.ToInt32(inputString);

WriteLine("Year in school? ");

inputString = ReadLine();

year = Convert.ToInt32(inputString);

if(credits > LOWCREDITS)

tuition = HOURFEE * credits;

else

if(credits == HIGHCREDITS)

tuition = FLAT;

else

tuition = FLAT + (credits - HIGHCREDITS) * RATE;

if(year < SENIORYEAR)

tuition = tuition - (tuition * DISCOUNT);

WriteLine("For year {0}, with {1} credits",

year, credits);

WriteLine("Tuition is {0}", tuition.ToString("C"));

}

}