UVa 11332 - Summing Digits

// UVa 11332 - Summing Digits

#include <iostream>
using namespace std;

#define datatype unsigned long long int

datatype sum_digs(datatype n) {
	datatype s = 0;
	while (n > 0) {
		s += n % 10;
		n /= 10;
	}
	return s;
}

int main() {
	datatype n;
	cin >> n;
	while (n) {
		while (n >= 10)
			n = sum_digs(n);
		cout << n << endl;
		cin >> n;
	}

	return 0;
}