#include<stdio.h>
#include<iostream>
#include<algorithm>
#include<vector>
#include<string>
#include<utility>
#include<map>
#include<set>
#include<queue>
#include<stack>
#include<functional>
#include<math.h>
#include<random>
#include <bitset>
#include <cassert>
#include <complex>
using namespace std;
//#define N (1000000000+7)
//#define N 998244353
#define INF 1e16
typedef long long ll;
typedef pair<ll,ll> P;
typedef pair<int,P> Q;


int main() {
    int n;
    cin>>n;
    while(true){
        int num = 0;
        while(n>=10){
            num+=n%10;
            n/=10;
        }
        num+=n;
        if(num<10){
            cout<<num<<endl;
            return 0;
        }
        n = num;
    }
}