#include <iostream>

using namespace std;

int main() {

unsigned long long int k = 0;

while (cin >> k) {

unsigned int result = 0;

if (k == 0) {

break;

}

while (true) {

if (result < 10 && k == 0) {

break;

}

else if (result > 9 && k == 0) {

k = result;

result = 0;

}

result += k % 10;

k /= 10;

}

cout << result << endl;

}

return 0;

}