#include <cstdio>
#include <iostream>
#include <string>
#include <cstring>
#include <algorithm>
#include <cmath>
#include <queue>
#include <vector>
#define PI acos(-1.0)
#define ms(a) memset(a,0,sizeof(a))
#define msp memset(mp,0,sizeof(mp))
#define msv memset(vis,0,sizeof(vis))
#define msd memset(dp,0,sizeof(dp))
using namespace std;
//#define LOCAL
string s;
int t=0;
void solve()
{
    for(string::iterator it=s.begin(); it!=s.end(); it++)
        t+=*it-‘0‘;
    s.clear();
    while(!(t%10==0&&t/10==0))
    {
        s.push_back((t%10)+‘0‘);
        t/=10;
    }
}
int main()
{
#ifdef LOCAL
    freopen("in.txt", "r", stdin);
    //freopen("out.txt","w",stdout);
#endif // LOCAL
    ios::sync_with_stdio(false);
    while(cin>>s&&*(s.begin())!=‘0‘)
    {
        solve();
        while(s.size()!=1)
        solve();
        printf("%c\n",s[0]);
    }
    return 0;
}