using System;
using Microsoft.Contracts;

public class BankAccount
{
    private decimal balance;
    invariant balance >= 0;

    public BankAccount()
        ensures balance == 0;
    {
        balance = 0;
    }

    public void Deposit(decimal amount)
        requires amount > 0;
        ensures balance == old(balance) + amount;
    {
        balance += amount;
    }

    public void Withdraw(decimal amount)
        requires amount > 0;
        requires amount <= balance;
        ensures balance == old(balance) - amount;
    {
        balance -= amount;
    }

    public decimal GetBalance()
        ensures result >= 0;
    {
        return balance;
    }
}

class Program
{
    static void Main()
    {
        BankAccount account = new BankAccount();
        account.Deposit(100.00m);
        account.Withdraw(30.00m);
        Console.WriteLine("Balance: $" + account.GetBalance());
    }
}