#include<iostream>
#include<string>
#include<algorithm>
#include<cctype>
#include<set>
#include<bitset>
#include<math.h>
#include<map>
#include<queue>
#include<iomanip>
using namespace std;


int main(){
    int n, ans;
    cin >> n;
    ans = n;
    for (int i = 0; i < 100; i++){
        n = ans;
        ans = 0;
        while(n){
            ans += n%10;
            n /= 10;
        }
        if (ans < 10){
            cout << ans << endl;
            return 0;
        }
    }
    cout << ans << endl;
}