Invalid expression "else"

I just got into programming , tried everything but the first else is always invalid, its probs real obvious but cant figure it out

if (answer == (num1 * num2)) 
{
    int responsegen = numbergenerator.Next (1, 4);

    switch (responsegen)  
    {

        case 1:
            Console.WriteLine ("Congrats! You're a natrual");
            break;

        case 2:
            Console.WriteLine ("Correct, Mini Einstein maybe?");

        default :
            Console.WriteLine ("Right answer , are you cheating?");

} 
else // this one right here 
{
    int diff = (answer - (num1 * num2));

    if (diff == 1) 
    {
        Console.WriteLine ("OHH so close you were only of by one number!");
    }

    if (diff <=10) 
    {
        Console.WriteLine ("Incorrect but you were close, only of by " + diff + "better luck next time!");                  
    }
}