public class Solution {
    public int AddDigits(int num) {
         if(num < 10){
            return num;
        }
        
        var n = num;
		
		while(true){
			var s = 0;
			while(n >= 10){
				s+= n % 10;
				n /= 10;
			}
			s += n;
			
			if(s < 10){
				return s;
			}
			n = s;
		}
    }
}