#include <bits/stdc++.h>
#define int long long
#define double long double
using namespace std;
const int MOD = 1000000007;
const int INF = 1e14;
using Graph = vector<vector<int>>;

signed main(){
  int N;
  cin >> N;
  while( N >= 10 ){
    int now = 0;
    while( N > 0 ){
      now += N%10;
      N /= 10;
    }
    N = now;
  }
  cout << N << endl;
}