<?php
    list($N) = explode(" ", trim(fgets(STDIN)));
    $sum = str_split($N);
    while(count($sum)!=1){
        $sum_0 = 0;
        for($i=0;$i>count($sum);$i++){
            $sum_0 += $sum[$i];
        }
        $sum = str_split($sum_0);
    }
    printf($sum[0]);