// See https://aka.ms/new-console-template for more information


long userInput = -1;

// Get the user input
while (userInput < 0)
{
    Console.WriteLine("Insert a natural number: ");
    // If the user input is in incorrect format, continue the cycle
    try
    {
        userInput = long.Parse(Console.ReadLine()!);
    }
    catch (Exception e)
    {
        userInput = -1;
    }
}

string numberAsString = userInput.ToString();


while (true)
{
    // If number has leading zeroes, remove them
    while (numberAsString.Length > 0 && numberAsString[0] == '0')
    {
        numberAsString = numberAsString.Remove(0, 1);
    }
    
    // If there is no number inside the variable, the user given number is impossible to make dividable by 3.
    if (numberAsString.Length == 0)
    {
        numberAsString = "-1";
        break;
    }
    
    
    // If number is dividable by 3, end the main cycle
    if (long.Parse(numberAsString) % 3 == 0)
    {
        break;
    }
    
    // Find the index of the first digit that's not dividable by 3.
    int index = -1;
    for (int i = 0; i < numberAsString.Length; i++)
    {
        if (numberAsString[i] % 3 != 0)
        {
            index = i;
            break;
        }
        
    }
    // Remove the digit from the whole number
    numberAsString = numberAsString.Remove(index, 1);

}

Console.WriteLine(numberAsString);