How to find all possible sums of all digits of integer?

Question

I need to find all possible sums of all digits of integer. Input: 234

Output: 36

Explanation: (2+3+2+4)+(3+2+3+4)+(4+2+4+3)

But I only know how to find the sum of the firs and last digit:

int num, sum = 0, firstDigit, lastDigit;

Console.Write("Enter any number: ");

num = Convert.ToInt32(Console.ReadLine());

lastDigit = num % 10;

firstDigit = num;

while (num >= 10)
{
num = num / 10;
}
firstDigit = num;


sum = firstDigit + lastDigit;

Console.WriteLine("Sum of first and last digit: " + sum);

Console.ReadLine();