#include <bits/stdc++.h>
#define rep(i,a,b) for(int i=a;i<=b;i++)
#define irep(i,a,b) for(int i=b;i>=a;i--)
#define pb push_back
#define endl "\n"

typedef long long int ll;


using namespace std;

void solve()
{
    int n;
    cin>>n;
    int root=0;
    while(n>0||root>9){
        if(n==0){
            n=root;root=0;
        }
        root=root+n%10;
        n=n/10;
    }
    
    cout<<root<<"\n";
    
}
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int t=1;
    // cin>>t;
    
    
    while(t--)
    {
        solve();
        
    
    }
    return 0;
}