#include<bits/stdc++.h>
#define int long long
using namespace std;
signed main(){
    int N;
    cin>>N;
    int n=N;
    while(n>=10){
        int cnt=0;
        for(int i=n;i>0;i/=10)cnt+=i%10;
        n=cnt;
    }
    cout<<n<<endl;
}