#include <iostream>
#include <vector>
using namespace std;
int main(){
    int N;
    cin >> N;
    for(int i = 0; i < 100; i++){
        int a = 0;
        while(N > 0){
            a += N % 10;
            N /= 10;
        }
        N = a;
        if(N / 10 == 0) break;
    }
    cout << N << endl;
}