#include<iostream>
#include<cstdio>
#include<string>
#include<string.h>
#include<cmath>
#include<map>
#include<vector>
#include<queue>
#include<algorithm>
using namespace std;

#define ll int
#define MAX 1005
#define inf 0x3fffff
#define vec vector<ll>

int main() {
	string s;
	while (cin >> s && s != "0") {
		ll res = 0;
		for (int i = 0; i < s.size(); i++)
			res += s[i] - '0';
		while (res >= 10) {
			s = to_string(res);
			res = 0;
			for (int i = 0; i < s.size(); i++)
				res += s[i] - '0';
		}
		cout << res << endl;
	}
}