#include <bits/stdc++.h>
using namespace std;
#define rep(i,n) for(int i = 0; i < (n);i++)
#define sz(x) int(x.size())
typedef long long ll;
typedef pair<int,int> P;
constexpr int INF = 1e9;

int main() {
  int n;
  cin >> n;
  auto calc = [&](int m){
    int res = 0;
    while (m) res += m % 10, m /= 10;
    return res;
  };

  while (n >= 10) {
    n = calc(n);
  }
  cout << n << endl; 
  return 0;
}