#include <bits/stdc++.h>
using namespace std;
int h(int n){
    int b=0;
    while(n!=0){
        b=b+n%10;
        n=n/10;
    }
    return b;
}
int main(){
    int n;
    cin>>n;
    int b=h(n);
    while(b>=10){
        b=h(b);
    }
    cout<<b<<endl;
}
/**************************************************************
	Problem: 1514
	User: panyuchen
	Language: C++
	Result: Accepted
	Time:9 ms
	Memory:2072 kb
****************************************************************/