// No.1168 Digit Sum Sequence
package main

import (
	"fmt"
	"strconv"
)

func main() {
	var n string
	fmt.Scan(&n)

	target := n
	for {
		sum := 0
		for _, r := range target {
			x, _ := strconv.Atoi(string(r))
			sum += x
		}
		target = strconv.Itoa(sum)
		if len(target) == 1 {
			break
		}
	}
	fmt.Println(target)
}