#include <bits/stdc++.h>
using namespace std;
 
int sum(int n){
    int s = 0;
    while(n != 0){
        s = s + n % 10;
        n = n / 10;
    }
    return s;
}
 
int main(){
    int n;
    cin>>n;
    int s = sum(n);
    while(s >= 10){
    	s = sum(s);
	}
	cout<<s<<endl;
}
/**************************************************************
	Problem: 1514
	User: admin
	Language: C++
	Result: Accepted
	Time:11 ms
	Memory:2072 kb
****************************************************************/