Recursive sum to find single digit

I have a number as N=2345

If I sum them the result will be 2+3+4+5 =14;

If I further sum them it will be 1+4 = 5; which is a single digit;

How to do this?

public static int FindSingleDigit(int N)
        {
            var sum = 0;
            int count = 0;
            while (N != 0)
            {
                sum = sum + (N % 10);
                N = N / 10;
                count++;
            }
            if (sum >= 10)  return FindSingleDigit(sum); 
            else return sum;
        }

Invoke it as

var result = FindSingleDigit(2345);