#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <queue>
#include <cmath>
#include <climits>
#include <iomanip>
#include <set>
#include <map>
using namespace std;
typedef long long ll;
int main(){
    ll n;
    cin >> n;
    while(1){
        string s;
        s = to_string(n);
        if(s.size() == 1){
            cout << s << endl;
            return 0;
        }
        ll now = 0;
        for(int i = 0;i < (int)s.size();i++)now += s[i] - '0';
        n = now;
    }
}