Cannot find a mistake

Question

Hello, I want to ask you to help me. My code should sum all the digits of inputted integer. However program prints something wrong. for example: integer 12 digits sum is 3; integer 24 digit sum is 6, but program prints 50 and 52. Can you help me? Here is the code:
Console.WriteLine("Input an integer: ");
string number = Console.ReadLine();
int sum = 0;
for (int i = 0; i < number.Length; i++)
{
sum =+ Convert.ToInt32(number[i]);
}
Console.WriteLine("Total sum of the digits is: " + sum);

Answer 1

Thanks for answer, Bennett Post. It works. I look for other function and found this:
sum += (int)char.GetNumericValue(number[i]);
It seems both of them works the same in this case.