#danglam228

#include <bits/stdc++.h>
using namespace std;
int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(0); cout.tie(0);
    freopen("MOTCS.INP", "r", stdin);
    freopen("MOTCS.OUT", "w", stdout);
    long long n;
    cin >> n;
    while (n >= 10) {
        long long res = 0;
        while (n > 0) {
            res += n % 10;
            n /= 10;
        }
        n = res;
    }
    cout << n << endl;
    return 0;
}