import java.io.*;

public class Main {
  public static void main(String[] args) throws IOException{
    BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
    String line=br.readLine();
    while(line.length()>1){
      int sum=0;
      for(int i=0;i<line.length();i++){
        sum+=(int)line.charAt(i)-48;
      }
      line=""+sum;
    }
    System.out.println(line);
  }
}