#define _GLIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;

int main() {
  int N; cin >> N;
  for(int i = 2; i <= 100; i++){
    int M = N, sum = 0;
    while(M > 0){
      sum += M%10;
      M /= 10;
    }
    N = sum;
    if(10 > N) break;
  }
  cout << N << endl;
}